Video Transcoding confusion

Heya.. I am trying to figure out "the best" transcoding settings - for me at least.
I thought.. Lets make sure that it is only H264 and AAC that doesn't need to be transcoded, and that all containers needs to be remuxed to MP4. And then of course the transcoding/encoding to the same H264 and AAC - this is at least what I can myself research should be compatible with more or less everything..

However, when I have it like this, some of my videos (already confirmed 15 out of 500) lags really badly.. Like it's only 3-5 FPS - the audio is fine though.

Am I doing something wrong?

If I have the same setup, just with the HEVC video codec also accepted to not be transcoded, everything seems to be fine.. That makes me think the original files are HEVC (H265, right?), but for my understanding that should NOT be able to just fine play on the browser.. So I am really confused. Also a lot more space is used up on the server. Does that make any sense??

Are there any way in Immich to see which type the file currently playing is and such?

I could of course just be fine and say whatever, it works.. But I really want to know why and what is happening haha.

Any helpers?