Probably the contents of your scene collection then, I'd imagine a blank one has no problems?
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So I tried it, created a new collection and only added game capture and game audio and still had the issue with just those 2 sources. Even tried that new collection in safe mode but still had this same issue.
A few observations I had though:
1. This frame drop issue is only happening in apex legends, no other game is causing this issue.
2. I did a sort of experiment where I launched my obs in safe mode first, then launched my game, had the same frame drop issue but then keeping my game running, I restarted my obs in normal mode. As my game was running on my main monitor, obs launched on my side monitor, no fps drop even though preview was on but as soon as i minimized my game and opened it again, i started having fps drop again.