The Xiaomi Redmi Note 9 is one of the most popular budget smartphones of 2020, but even it isn’t immune to performance issues.If your device starts to “brake,” hang when opening apps, or respond slowly to touch, it doesn’t always mean it’s time to buy a new phone. 80% of the time lags are caused by software failures that can be fixed on your own.
We analyzed hundreds of user complaints on forums (including 4PDA, Mi Community, and Reddit) and identified 10 key reasons why the Redmi Note 9 (M2003J15SC/M2003J15SS) is starting to slow down, from trivial cache overflow to hardware issues with the MediaTek Helio G85 chip — take everything in order, with diagnostic and repair instructions.
Important: If the lags appeared after upgrading to MIUI 14+ based on Android 12/13, the problem may lie in unoptimized software for older models.
1. Overloading of RAM (RAM)
The Redmi Note 9 is equipped with 3-6 GB of RAM (depending on the version), but even this may not be enough when used actively. MIUI aggressively manages memory: closes the background processes, but sometimes forgets to free up resources, which leads to hanging.
How to check:
- 📱 Open Settings → About Phone → Memory and see how much RAM is left. If less than 500MB is critical.
- 🔄 Run Task Manager (swipe up with retention) and see which apps consume the most memory.
- 📊 Use the CPU Monitor app (from Google Play) to analyze the CPU’s real-time boot.
Decisions:
- ⚡ Close all background applications manually through Task Manager.
- 🗑️ Clear the cache of system and user applications (Settings → Applications → Select App → Storage → Clear the cache).
- 🔧 Turn off auto-run of unnecessary programs: Settings → Permissions → Auto-run.
- 🛠️ Install Lite versions of apps (e.g. Facebook Lite, Messenger Lite).
⚠️ Warning: Don’t use Google Play’s memory boosters — they often make the lags worse, putting additional strain on the processor.
2. Completed storage (internal memory)
If the phone has less than 10% of its total space, it starts to run slower, because Android uses some of the memory for temporary files and cache. On the Redmi Note 9 with 64/128 GB, this threshold is about 6-12 GB of free space.
How to check:
- 📁 Go to Settings → Storage and see the usage chart.
- 🔍 Pay special attention to the System Data and Other sections, which are often inflated by debris.
Decisions:
| Problem. | How to fix it | Tools |
|---|---|---|
| Garbage files | Cleaning through Security → Cleaning | Built-in MIUI |
| Duplicate photo/video | Use Google Files or CCleaner | Third-party annexes |
| Cash app. | Manual cleaning for each application | Settings → Applications |
| Large files (>500 MB) | Transfer to SD- card or cloud | Mi Cloud, Google Drive |
If you have left the lags after cleaning, try formatting your phone as new (reset to factory settings). pre-reverse through Settings → Additional → Backup and Reset.
Delete unnecessary photos and videos|Transfer files to SD-card|Save contacts in a Google account|Write down passwords from apps|Untie the Mi Account from the device-->
Processor overheating (trottling)
The MediaTek Helio G85 in the Redmi Note 9 is prone to overheating under prolonged loads (games, video editing, charging + use). When the chip temperature exceeds 60-65 ° C, trottling is triggered, a forced decrease in performance for cooling, which causes sharp lags and friezes.
Signs of overheating:
- 🌡️ The phone body is hot (especially in the upper part near the camera).
- 🎮 Lags in games (PUBG Mobile, Genshin Impact) in 15-20 minutes of gameplay.
- 🔋 Fast battery discharge (more than 10% in 30 minutes in idle).
- ⚡ Unintentional charging shutdown (if temperature exceeds 70°C)
Decisions:
- 🛑 Close all background apps before playing.
- 🧊 Remove the case during heavy use.
- 🔄 Reduce graphical settings in games (e.g., in PUBG Mobile, select Balanced or Low Settings).
- 🔧 Turn off autobrightness and set the value manually to 50-70%.
- 🛠️ Replace the heat paste (if the phone has been disassembled before).
⚠️ Warning: If the phone overheats even in a simple (for example, when watching YouTube), this may indicate a battery malfunction or a short circuit on the motherboard, in which case a diagnosis is required at the service center.
How to check the temperature of the processor?
4. Firmware problems (MIUI)
Xiaomi regularly releases updates for the Redmi Note 9, but not all are optimized. Users often complain about the lags after upgrading to:
- MIUI 12.5 (Android 11) – problems with animations and delays when opening applications.
- MIUI 13/14 (Android 12/13) – increased power consumption and overheating.
How to check the software version:
- Open the Settings. → The phone.
- Check out the MIUI version and the Android version.
- Compare it to the latest stable builds on Xiaomi’s official website.
Decisions:
- 🔄 Reverse to the previous version MIUI (if the lags appeared after the update). This requires unlocking the bootloader and flashing the phone through Fastboot.
- 🛠️ Install custom firmware (e.g. Pixel Experience or LineageOS), but this voids the warranty.
- 🔧 Reset to factory settings (Settings → Additional → Backup and Reset → Resetting).
If you are not sure about your skills, it is better to contact the Xiaomi service center for flashing. The cost of the service is from 1,500 rubles.
💡
Before rolling back the firmware, be sure to back up your data through Settings → Additional → Backup and Reset → Local Backup. It will not be possible to recover data after the reset!
5. Battery malfunction
The 5020 mAh battery in the Redmi Note 9 is designed for 500-800 charge cycles, and after 2-3 years of use, its capacity drops to 60-70%, resulting in:
- ⚡ Fast discharge (less than 4 hours of screen time).
- 🐢 Slowed operation at charge levels below 20%.
- 🔌 Spontaneous reboots when charging is connected.
How to check the status of the battery:
- 📊 Install AccuBattery (from Google Play) and analyze the discharge graph.
- 🔧 Enter ##4636## in the dialer → Select Battery Information → See the wear level.
Decisions:
- 🔋 Replace the battery at the service center (cost is 2,000-3,500 rubles).
- 🔌 Use only original chargers (18W for the Redmi Note 9).
- 🔄 Turn off fast charging in settings if the phone overheats while charging.
⚠️ Warning: If the battery is swelling (visible on a raised screen or body), immediately turn off the phone and call the service. Risk of fire!
6. Viruses and malware
Even if you haven’t installed suspicious apps, viruses can get in through:
- 📎 Investments in WhatsApp/Telegram.
- 🌐 Sites with pirated content (movies, games, programs).
- 📲 APK- files from unverified sources.
Signs of infection:
- 📱 Appearance of unfamiliar apps on the menu.
- 💸 Debiting money from the account (SMS to short numbers).
- 🔋 Fast battery discharge even in simple.
- 📶 Increased mobile internet traffic.
How to check and clean:
- Install Malwarebytes or Dr.Web Light from Google Play.
- Run a full system scan.
- Remove any threats you find.
- Check the list of installed applications (Settings → Applications) for suspicious programs (for example, with names such as System Update, Flash Player).
If viruses are not detected but the lags remain, try:
- 🔧 Disable installation from unknown sources (Settings → Privacy → Special Permits).
- 🛡️ Enable Google Play Protect (Play Store → Settings → Play Protect).
💡
If the phone still brakes after removing the viruses, reset to factory settings. Viruses can be embedded in system files, and a routine cleaning doesn't help.
Hardware problems (motherboard, plumes)
If all software methods did not help, the cause of the lags may lie in the iron:
- 🔌 Oxidation of contacts on the motherboard (due to moisture ingestion).
- 🔧 Memory chip dump (if phone falls)
- 📶 Display plume problems (lead to delays in touching).
Diagnosis:
- 🔍 Check if there were any physical injuries (falls, impacts, water inhalation).
- 🎧 Connect headphones – if the sound is interrupted, it could indicate an audio chip problem.
- 📱 Check the touchscreen: If the screen reacts to late touches or misses taps, the plume may wear out.
Decisions:
- 🛠️ Contact the service center for the diagnosis of the motherboard (cost - from 1000 rubles).
- 🔧 If the phone falls, check for microcracks on the board (you can see them under a microscope).
- 💦 If the phone has been in contact with water, an ultrasonic cleaning of the board is required.
⚠️ Warning: Self-repairing a motherboard without experience can cause your phone to fail completely. If you're not sure, trust the professionals.
8. Memory Card Conflicts (SD)
Many Redmi Note 9 users install microSDs to expand memory, but cheap or damaged cards can cause:
- 📁 Slowly write/read files.
- 🔄 Hanging when opening applications installed on SD.
- 💥 Memory card is corrupted.
How to check:
- 📊 Connect the card to your PC and run error check (Properties → Service → Check in Windows).
- 🔧 Check card speed with the SD Card Tester (from Google Play). Minimum speed for normal operation is 10MB/s.
Decisions:
- 🗑️ Format the card in exFAT format (via PC).
- 🔄 Buy a class card A1 or A2 (e.g. SanDisk Ultra or Samsung EVO).
- 🚫 Do not install applications on the SD- card, this always slows down.
💡
If you use a SD- card as internal memory (Adoptable Storage feature in Android), disable this option. It greatly reduces performance due to slow read/write speeds.