MacBook Pro EXTREMELY slow after Upper Case Assembly replacement

I just replaced my whole upper case on my MacBook and it has been unbearably slow ever since.

I made sure I followed the guide properly and I also cleaned out and reapplied my thermal compound for my CPU. I also mounted the heatsink tightly so it has enough pressure.

I have tried doing a clean install of macOS (full drive wipe), resetting the NVRAM, PRAM, and SMC but there's still a lot of lag constantly.

There would be around 8-10 seconds of lag between keystrokes or starting Safari.

GeekBench 4 results for my CPU are: 415 Single core and 1396 multi core, which is way less than the average according to the comparison chart.

Third-party softwares are telling my that my battery health is at 97% and that my temps for CPU and all other internal components are below 34 degrees Celsius.

What could it be?