1.15.2 is an early release, its not guaranteed to be perfectly optimized, also, it could also be something on your end causing the issue.
Even a "God pc" Can get lower fps based on how the resources on your computer are being shared, there could be other programs using up too much resources and minecraft cant keep up.
I recommend just switching back to 1.14 until 1.15 reaches its final release (most servers haven't even upgraded to 1.13 yet)
On a personal note, 1000 fps isnt really a thing, sorry bud, and the human eyes see at about 30fps on average (based on online studies), so your 50-100 fps and "insane" stuttering, that shouldnt exist. I think your overreacting a bit...
This is an intended feature.
Please research something before posting it to the BUG reporting site. If you want the feature removed, suggest it on the suggestion forum (On reddit, r/minecraftsuggestions)
You need more information, and this isnt the place to post this. This is most likely something wrong on your end, not minecraft, post the console log on r/minecraft on reddit with the correct flair or whatnot
or just post it somewhere else besides the bug tracking site..
Is there any way we can select what entity activates the command block then? This is really important to me...
Perhaps it was just lag on my side? I wasnt using any mods.... (I cant even use mods on 1.14 anyways) and it was just a simple command block change in the block
yet it did that whole "not activating/deactivating for like 1-2 minutes" thing
Thats just a mix of sprinting and walking, dont press it 3 times, to sprint, either hold control (or whatever your sprint key is) or just double tap W
thats been happening for years 😛