You can try my fix out.
No telling if it’ll work for you but it did work for me. I made a thread about it (https://us.forums.blizzard.com/en/d3/t/my-solution-to-lagstuttering-etc/34340)
I, like many others, was having intermittent performance issues, lag, stutter, etc. I have an older gaming laptop from 2015 but should be able to run this with minimal problems but in reality that was not the case.
I searched on the forums here and found many posts regarding the issue being something to do with FMOD. People getting FMOD errors in logs and such. Then I saw another post from a fricken wizard who determined that the issues weren’t with FMOD itself but with outdated MSVC++ files. It seems the game is using 2015 version of these MSVC++ files. The latest version is 2019(mine was from nov 2020).
This poster suggested replacing these files but didn’t explain how to replace them. However, they are really easy to replace. If you look in your windows folder under either the sysWOW64 or system32 folders(depending on the bit version of the game you are running) search for msvcp140.dll and vcruntime140.dll. Once you find these files copy them directly into the diablo3 folder(x64 if using 64 bit version), overwriting the files that are there.
Also I would suggest making a shortcut and adding the -launch parameter so battle.net doesn’t load upon start. It is possible if the game is started using battle.net launcher it may try to “repair” the files that were just overwritten negating everything that was just done.
I have done these steps and can confirm I have had 0 issues besides usual PC lag and rando disconnects.