[Redmi K20 / Xiaomi Mi 9T Questions & Answers] GenomeX:
1st) MIUI optimization not very helpful!

2nd) that adb commend effect a little (but fix many off lags!)

adb shell cmd package bg-dexopt-job

but i got 8-9 hrs SOT on MIUI 11 and 7-8 hrs on MIUI 12

for beggining...I uninstall some apps.

MIUI bloatwares:

adb shell pm uninstall -k --user 0 com.miui.mishare.connectivity
adb shell pm uninstall -k --user 0 com.miui.bugreport
adb shell pm uninstall -k --user 0 com.miui.miservice
adb shell pm uninstall -k --user 0 com.miui.huanji
adb shell pm uninstall -k --user 0 com.android.browser
adb shell pm uninstall -k --user 0 com.mi.globalbrowser
adb shell pm uninstall -k --user 0 com.xiaomi.midrop
adb shell pm uninstall -k --user 0 com.android.midrive
adb shell pm uninstall -k --user 0 com.mipay.wallet
adb shell pm uninstall -k --user 0 com.mipay.wallet.id
adb shell pm uninstall -k --user 0 com.mipay.wallet.in
adb shell pm uninstall -k --user 0 com.xiaomi.payment
adb shell pm uninstall -k --user 0 com.xiaomi.mipicks
adb shell pm uninstall -k --user 0 com.xiaomi.glgm
adb shell pm uninstall -k --user 0 com.xiaomi.mircs
adb shell pm uninstall -k --user 0 com.miui.virtualsim
adb shell pm uninstall -k --user 0 com.xiaomi.mipicks
adb shell pm uninstall -k --user 0 com.xiaomi.scanner
adb shell pm uninstall -k --user 0 com.miui.smsextra
adb shell pm uninstall -k --user 0 com.miui.msa.global
adb shell pm uninstall -k --user 0 com.miui.systemAdSolution
adb shell pm uninstall -k --user 0 com.miui.touchassistant
adb shell pm uninstall -k --user 0 com.miui.personalassistant
adb shell pm uninstall -k --user 0 com.miui.nextpay
adb shell pm uninstall -k --user 0 com.miui.visimcore
adb shell pm uninstall -k --user 0 com.miui.yellowpage
adb shell pm uninstall -k --user 0 com.miui.miwallpaper
adb shell pm uninstall -k --user 0 com.miui.hybrid
adb shell pm uninstall -k --user 0 com.miui.hybrid.accessory
adb shell pm uninstall -k --user 0 com.miui.freeform
adb shell pm uninstall -k --user 0 com.miui.fm
adb shell pm uninstall -k --user 0 com.miui.fmservice
adb shell pm uninstall -k --user 0 com.mi.android.globalpersonalassistant
adb shell pm uninstall -k --user 0 com.mi.android.globalminusscreen
adb shell pm uninstall -k --user 0 com.facebook.services
adb shell pm uninstall -k --user 0 com.facebook.system
adb shell pm uninstall -k --user 0 com.facebook.appmanager
adb shell pm uninstall -k --user 0 com.facebook.katana
adb shell pm uninstall -k --user 0 com.dsi.ant.server
adb shell pm uninstall -k --user 0 com.netflix.partner.activation

Google bloatwares:

adb shell pm uninstall -k --user 0 com.google.android.printservice.recommendation
adb shell pm uninstall -k --user 0 com.google.ar.lens
adb shell pm uninstall -k --user 0 com.google.ar.core
adb shell pm uninstall -k --user 0 com.google.android.marvin.talkback
adb shell pm uninstall -k --user 0 com.google.android.projection.gearhead
adb shell pm uninstall -k --user 0 com.google.android.music
adb shell pm uninstall -k --user 0 com.google.android.videos
adb shell pm uninstall -k --user 0 com.google.android.apps.photos
adb shell pm uninstall -k --user 0 com.google.android.apps.tachyon
adb shell pm uninstall -k --user 0 com.google.android.apps.books
adb shell pm uninstall -k --user 0 com.google.android.apps.docs

Android bloatwares:

adb shell pm uninstall -k --user 0 com.android.providers.downloads.ui
adb shell pm uninstall -k --user 0 com.android.stk
adb shell pm uninstall -k --user 0 com.android.hotspot2
adb shell pm uninstall -k --user 0 com.android.managedprovisioning
adb shell pm uninstall -k --user 0 com.android.cts.priv.ctsshim
adb shell pm uninstall -k --user 0 com.android.cts.ctsshim
adb shell pm uninstall -k --user 0 com.android.apps.tag
adb shell pm uninstall -k --user 0 com.android.dreams.basic
adb shell pm uninstall -k --user 0 com.android.dreams.phototable
adb shell pm uninstall -k --user 0 com.android.bluetoothmidiservice
adb shell pm uninstall -k --user 0 com.android.bookmarkprovider
adb shell pm uninstall -k --user 0 com.android.smspush
adb shell pm uninstall -k --user 0 com.android.storagemanager
adb shell pm uninstall -k --user 0 com.android.internal.systemui.navbar.twobutton
adb shell pm uninstall -k --user 0 com.android.internal.systemui.navbar.threebutton
adb shell pm uninstall -k --user 0 com.android.internal.systemui.navbar.gestural
adb shell pm uninstall -k --user 0 com.android.internal.systemui.navbar.gestural_narrow_back
adb shell pm uninstall -k --user 0 com.android.internal.systemui.navbar.gestural_wide_back
adb shell pm uninstall -k --user 0 com.android.internal.systemui.navbar.gestural_extra_wide_back
adb shell pm uninstall -k --user 0 com.android.theme.font.notoserifsource
adb shell pm uninstall -k --user 0 com.android.theme.icon.roundedrect
adb shell pm uninstall -k --user 0 com.android.theme.icon.square
adb shell pm uninstall -k --user 0 com.android.theme.icon.squircle
adb shell pm uninstall -k --user 0 com.android.theme.icon.teardrop
adb shell pm uninstall -k --user 0 com.android.theme.color.black
adb shell pm uninstall -k --user 0 com.android.theme.color.cinnamon
adb shell pm uninstall -k --user 0 com.android.theme.color.green
adb shell pm uninstall -k --user 0 com.android.theme.color.ocean
adb shell pm uninstall -k --user 0 com.android.theme.color.orchid
adb shell pm uninstall -k --user 0 com.android.theme.color.purple
adb shell pm uninstall -k --user 0 com.android.theme.color.space
adb shell pm uninstall -k --user 0 com.android.theme.icon_pack.circular.themepicker
adb shell pm uninstall -k --user 0 com.android.theme.icon_pack.circular.settings
adb shell pm uninstall -k --user 0 com.android.theme.icon_pack.circular.systemui
adb shell pm uninstall -k --user 0 com.android.theme.icon_pack.circular.launcher
adb shell pm uninstall -k --user 0 com.android.theme.icon_pack.circular.android
adb shell pm uninstall -k --user 0 com.android.theme.icon_pack.filled.themepicker
adb shell pm uninstall -k --user 0 com.android.theme.icon_pack.filled.settings
adb shell pm uninstall -k --user 0 com.android.theme.icon_pack.filled.systemui
adb shell pm uninstall -k --user 0 com.android.theme.icon_pack.filled.launcher
adb shell pm uninstall -k --user 0 com.android.theme.icon_pack.filled.android
adb shell pm uninstall -k --user 0 com.android.theme.icon_pack.rounded.settings
adb shell pm uninstall -k --user 0 com.android.theme.icon_pack.rounded.systemui
adb shell pm uninstall -k --user 0 com.android.theme.icon_pack.rounded.launcher
adb shell pm uninstall -k --user 0 com.android.theme.icon_pack.rounded.android
adb shell pm uninstall -k --user 0 com.android.internal.display.cutout.emulation.double
adb shell pm uninstall -k --user 0 com.android.internal.display.cutout.emulation.tall
adb shell pm uninstall -k --user 0 com.android.internal.display.cutout.emulation.corner

This apps can not uninstall (or not safe) but can disable:

adb shell pm disable-user com.android.traceur
adb shell pm disable-user com.android.egg
adb shell pm disable-user com.miui.android.fashiongallery
adb shell pm disable-user com.facemoji.lite.xiaomi

EXTRA:

- Disable Auto Sync

- Disable Dual SIM on 4G

- Disable AOD (Optional)

- Disable Notification Wake Screen (Optional)

- Disable Raise To Wake (Optional.. for me this option is enabled)

- Turn off Wifi and bluetooth scan for location service

- Disable Google Assistant in Lockscreen

- Disable "Update at night automatical" in Weather app

- Set restrict mode for some apps you not use all of day

- Set timer for Dark mode, Battery saveing mode. (I set dark from 8pm-8am, battery saving from 00-8am)

- Unlock bootloader and install a AOSP base ROM :D (MIUI very lagy and battery hungre custom skin in android world!!!)