Why is KNLMeansCL slowing me down so much

[Selur]

It's not an interlaced source. I go onto the (De-)Interlace tab to set Auto Deinterlace Handling to None.

That is not needed. Since the source isn't detected as interlaced Hybrid won't deinterlace.

my fps drops from nearly 4fps to 0.3fps.

tried the script, KNLMeansCL isn't the bottleneck for me.
(I'm using a Ryzen 9 3850x)
with the script as you posted it, I get 19.5fps
(just) without KNLMeansCL I get 20.5fps
(just) without daamod I get 27fps
(just) without FineDeHalo I get 34.4fps

Assuming you are correct and it is indeed KNLMeansCL that is slowing you down and it didn't do this before, I would suggest to:
a. check the taskmanager whether some anti virual tool or similar is slowing the whole process down
b. reinstall your GPU drivers

Cu Selur