Why is KNLMeansCL slowing me down so much

[Selur]

Is there a way to update this filter or figure out why it's suddenly causing problems?

Not enough details to adjequately comment this.
You neither show the VapourSynth nor AviSynth scripts you use, nor do you provide any details about the source.

Wild guess - from the speed drop - would be that before your source was progressive and you are now using an interlaced source which needs deinterlacing.

-> Read the sticky, provide a debug output and detailed steps what you do and I can look at it after work.

Cu Selur