The total number of samples the renderer is making per pixel per frame is equal to “Path Trace Samples Per Pixel” * “Subframes per frame”. You have those set to 128 and 5 respectively. You can increase the subrames by some multiple and decrease the samples per pixel by the inverse to maintain the same render time an quality, but increase the motion blur quality.
Try settings the samples per pixel to 8 and subframes to 80.