I wanted to chime in on this issue as I managed to solve it. I recently got an RTX 3080 (Gigabyte Aorus). Previously had a 2080 with zero issues in OBS. Once I upgraded to the 3080, I immediately started having massive lag issues in OBS. OBS was just simply open and idling, which led to 80-99% GPU usage. Whereas with the 2080 right before the video card swap, OBS would idle around 10-25% GPU usage which is normal for me because I use 6 cameras + 550+ scenes. Yes, I am insane. Moral of the story is, 2080 I had no issues. Swapped in the 3080 video card, immediately had max GPU usage issues during an idle OBS.
I went through all the internet forums/reddits to find others having this same issue and not able to solve it through all the basic troubleshooting steps (run OBS as admin mode, latest drivers, enable game mode, HAGS, Xbox game bar, etc. etc.).
TL;DR My solution? I had to reseat the video card. Yes it sounds insanely simple and I do not get it, but this somehow solved the problem.
Longer description: I thought perhaps the PCI E slot was having problems so I wanted to try out different slots. I had to rip out all my other PCI cards (USB expansion card and MSI 710 for additional monitors, yes I have 5 monitors because I'm insane). Right after ripping out the other cards and swapping slots, the 3080 was purring along fine with an open and idle OBS just like the 2080 was with GPU usage idling around 10-25% (normal for my OBS).
I started to swap everything back to the original configuration with readding cards back in (including putting the 3080 back into the original PCI E slot) and it was still working. It was as if the bad nightmare never even happened and the 3080 no longer had issues. (Only other possible accidental solution was I did swap USB cables around unintentionally because I'm pretty sure I did not 100% put all USB cables back in their original configuration, but I doubt this caused the 99% GPU usage while OBS idled).
I don't get it, but I won't argue with the results. I hope this might help someone else out in the future who may have a similar problem.
ADDITIONAL INFO:
-Running a simple 3d FPS game did not send my GPU to 99% usage
-I did plenty of reboots
-OBS was idling with zero streaming/recording going on, yet ramping up to 80-99% GPU usage
-Swapping to a "fresh" OBS with zero scenes did bring down GPU usage a lot, but when I started adding cameras, GPU usage was starting to shoot up fast (roughly 5-20% GPU usage per camera added with Elgato Cam Link CamLink being the worst offender)
FULL SPECS:
GIGABYTE RTX3080 AORUS MASTER
AMD Threadripper 1950x
ASUS PRIME X399-A AMD Threadripper TR4 DDR4 M.2 U.2 X 399 EATX HEDT Motherboard
G.SKILL 32GB (4 x 8 GB) DDR4 3000MHz
1200W PSU
Intel Solid State Drive (SSD), 660P Series, 1 TB