Why avisynth bottlenecks ffmpeg nvenc-oding ?

Hi,

Transcoding of a H.264 file with my RTX2060 goes with a blazingly fast ~400 fps, when decoding is hw. accelerated too.

The same thing when served with AviSynth FFmpegSource2 (AviSynth+ 3.4 (r2923, 3.4, x86_64)) falls down to ~130fps.

Now I understand, that frameserving+decoding is a CPU intense job, and my CPU is kind of a relic (Xeon E5450), but Avsmeter says:

FPS (min | max | average): 121.2 | 251.4 | 196.3
Process memory usage (max): 47 MiB
Thread count: 9
CPU usage (average): 87.9%

This significant difference between 130 and 196 is the problem, I am trying to track down. Any idea please?