The difference between performance on 1st match vs 2nd and forward

My first match on startup with a PC that is on the minimum system requirements is:
-Stable and consistent, buttery smooth 60 fps even on the most action-packed areas (Big team battle with like 20 people on one objective included).
-No stutters, input lag, drops, nothing.
-Able to slightly crank up the settings and be able to play at native resolution.
-Able to use stuff like Discord on the background without problems.

My second match (and forward), however, is:
-Getting 45 fps at MOST at the least intensive areas possible on 4v4 with all the settings cranked down as low as possible AND the resolution dropped as low as possible.
-Constant stutters, lag and drops.
-Discord drops it even further.
-Big team battle is unplayable.

I genuinely don’t understand how this works. Not saying that to judge 343 or anything, I just don’t get it. How does that happen? Any fixes?

A memory leak maybe?