Warping performance: inconsistent GPU Usage

i’m developing my warping app under Ubuntu 20.04.2 LTS with videocard P1000 and driver v.460.91.03.
I noticed a strange behaviour in GPU usage, In facts if i connect only 1 or 2 monitor 1920x1080@60Hz as soon as i enable warping (XNVCTRLSetScanoutWarping) with only 6 vertices (= 2 triangles) the GPU usage instantly go to 20%.
But if i connect all 4 video outputs to 4 monitors 1920x1080@60Hz and i do warping on first 2 monitors the GPU usage go down to 5%.
This happens only in Ubuntu, i tested the same sample under Windows and the GPU usage is coerent and it is about 3% for each video ouput warped.
I’ve tried also drivers 495.46 and the new beta 510.39.01 but the performance problem remains.

In this table i recap all my tests:

Any advice? Thank you