I also use this phone and doesn't seem to have any issue, and I have over 700 entires in my bitwarden vault. My main browser is firefox though. If the issue is bitwarden is slow to unlock the vault, this is normal if your vault has high KDF iterations. Also, if you're using the stock os, mi ui is very aggressive at killing background tasks, which means bitwarden will need to be relaunched over and over again and have to unlock the vault again every time, which further increase the perception of slowness.
Redmi note 8
I also use this phone and doesn't seem to have any issue, and I have over 700 entires in my bitwarden vault. My main browser is firefox though. If the issue is bitwarden is slow to unlock the vault, this is normal if your vault has high KDF iterations. Also, if you're using the stock os, mi ui is very aggressive at killing background tasks, which means bitwarden will need to be relaunched over and over again and have to unlock the vault again every time, which further increase the perception of slowness.
It has problems on the stock MIUI ROM. I have the same phone, I have experienced it. The autofill feature used to turn off automatically.
I installed the official LineageOS, some time ago. Now, it works great, no problems at all.