TL;DR - disable any software which limits your bandwidth. It should be a no-brainer, but it took me a long while to figure out why I was having frame drop issues. Doh.
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I thought I'd share my most recent finding with OBS. Maybe someone has the same issue I've been having and can benefit from it.
Since the very beginning of using OBS, I've been having random frame drop issues. Sometimes I'd have none at all, sometimes I'd barely hit a 1k bitrate and have 20-30% frame drops - which makes a stream completely unwatchable.
So I looked into the issue, did the server testing as described here: https://obsproject.com/forum/threads/dropped-frames-disconnecting-lag-read-this-first.8870/
as well as constantly fiddling with bitrate, fps, resolution and god knows what else, but to no avail.
Then I remembered that before I got my current isp with 4g internet, I had to share my gf's connection, which is crappy beyond belief. In order to not hog the entire bandwidth of the connection, I installed a piece of software where I could manually limit how much of the total bandwidth I was using. Needless to say, when I got my new connection I never had to use the software again, so I forgot about it completely.
Then I was looking at what processes were running in connection with something completely unrelated, and discovered that the bleeping limiting software had a "helper" process running, even though I hadn't activated it. So I uninstalled the software completely and hey presto, no frame drop issues whatsoever. Currently streaming at 60fps with 5k bitrate with a frame drop rate of 0. As in not a single frame dropped during a four-hour stream.