[Redmi 9A ROMs, Kernels, Recoveries, & Other Develo] Xcuse:
If you want the best performance (as far as I think) you should read this and follow it.

I flashed it yesterday and everything worked fine but was a bit laggy.

Phone has 2GB ram which might also be a reason for all that.

Used the mentioned Lineage with Gapps.

Used this TWRP : TWRP-3.4.2B-0216-REDMI9A-CN-wzsx150-fastboot.zip

And this MIUI : V12.0.19.0.QCDMIXM (Flashed manually since device was on 12.0.14

Also wiped everything before.

Then was sick of it (got even more laggy and slow) and did some research. I think the below mentioned MIUI stock is better to start off from.

Removed Pin lock and then went on.

ALSO I used those guides to wipe the phone effectively and adjusted the setting with rm-rf instead of formatting (you need to have TWRP installed already)

So to make it short I used ONLY these steps from the guides below

-Go to Settings , make sure (Use rm-rf instead of formatting) is unthicked

-!Now go back and go to Wipe ,then Advanced Wipe, select only System and Swipe to Wipe

-Wait until it finished , it could take lot of time (About 20 Minutes), have a cup of tea while you wait :D .

-Repeat the System Wipe for at least 20 times (More or less depend on users, but to be safe do it 20 or more ).

-Then It's done you can install any ROM you want, after installing the new ROM go check your Write Speed again with

//

-Untick "Use rm -rf instead of formatting" in TWRP settings (again, VERY IMPORTANT)

-Do a full wipe (format data, system, cache) (why don't we need to wipe data, dalvik cache in advanced wipe? because we have already done those steps when format data)

https://xdaforums.com/t/how-to-fix-lags-slowness-due-to-emmcs.3745047/

https://xdaforums.com/t/rom-6-0-1-b...laxy-core-prime.3407171/page-20#post-67824939

(In addition) to that I first went for a format through the TWRP, then the advanced options and then 15-20 Factory (swipe) wipes (they are described also in the second bullet point above with the !)

For the advanced options I picked cache, dalvik and internal storage / data.

Then first flashed this stock MIUI : V12.0.20.0.QCDMIXM

Let the system boot up maybe sign in and click on the button to start the process of MIUI12, as soon as there is written "loading apps" at the bottom: hold volume down and lock button until it shuts down, then immediatly hold volume up and lock button to boot into TWRP.

Did the same formatting steps AGAIN.

Then secondly flashed the same TWRP with the new vbmeta.img

By this but without root

https://xdaforums.com/t/guide-twrp-...ment-of-twrp-with-the-stock-recovery.4605351/

Then flashed the same MIUI again.

DO NOT BOOT IT UP!!!! stay in fastboot

Also had to use the fastboot reboot fastboot command which didnt boot into fastbootD but to the TWRP (which I assume is normal)

(You might consider checking if the device is still unlocked here by using the unlock tool. Read somewhere that it can happen that it locked again but wasn't in my case - so maybe you can leave that out)

THEN AGAIN THE FORMATTING STEPS.

Then flashed the LineageOS again and it seems to work WAAAAY better. Not as many lags before and just overall smoother experience.

AGAIN FORMATTING. Then boot up and let it chill.

Did a benchmark and got around 10-20% better results after flashing.

So my Advise: Definity use those ways to wipe the phone and THEN proceed with the V12.0.20.