Things started well but now .19 FPS and slowing

[Selur]

# Imports
import vapoursynth as vs
# getting Vapoursynth core
import sys
import os
core = vs.core
# Import scripts folder
scriptPath = '/home/jellyfin/opt/hybrid/vsscripts'
sys.path.insert(0, os.path.abspath(scriptPath))
# Import scripts
import srestore
import qtgmc
import validate
# Source: '/working/raw/VENTURE_BROS_VOL_1_DISC_1/C1_t01.mkv'
# Current color space: YUV420P8, bit depth: 8, resolution: 720x480, frame rate: 29.97fps, scanorder: top field first, yuv luminance scale: limited, matrix: 470bg, transfer: bt.601, primaries: bt.601 ntsc, format: mpeg-2
# Loading /working/raw/VENTURE_BROS_VOL_1_DISC_1/C1_t01.mkv using LWLibavSource
clip = core.lsmas.LWLibavSource(source="/working/raw/VENTURE_BROS_VOL_1_DISC_1/C1_t01.mkv", format="YUV420P8", stream_index=0, cache=0, fpsnum=30000, fpsden=1001, prefer_hw=0)
frame = clip.get_frame(0)
# setting color matrix to 470bg.
clip = core.std.SetFrameProps(clip, _Matrix=vs.MATRIX_BT470_BG)
# setting color transfer (vs.TRANSFER_BT601), if it is not set.
if validate.transferIsInvalid(clip):
  clip = core.std.SetFrameProps(clip=clip, _Transfer=vs.TRANSFER_BT601)
# setting color primaries info (to vs.PRIMARIES_BT470_BG), if it is not set.
if validate.primariesIsInvalid(clip):
  clip = core.std.SetFrameProps(clip=clip, _Primaries=vs.PRIMARIES_BT470_BG)
# setting color range to TV (limited) range.
clip = core.std.SetFrameProps(clip=clip, _ColorRange=vs.RANGE_LIMITED)
# making sure frame rate is set to 29.97fps
clip = core.std.AssumeFPS(clip=clip, fpsnum=30000, fpsden=1001)
# making sure the detected scan type is set (detected: top field first)
clip = core.std.SetFrameProps(clip=clip, _FieldBased=vs.FIELD_TOP) # tff
# Deinterlacing using QTGMC
clip = qtgmc.QTGMC(Input=clip, Preset="Fast", TFF=True) # new fps: 59.94
# Making sure content is preceived as frame based
clip = core.std.SetFrameProps(clip=clip, _FieldBased=vs.FIELD_PROGRESSIVE) # progressive
# adjusting frame count and rate with sRestore
clip = srestore.sRestoreMUVs(source=clip, frate=29.9700)
# adjusting output color from: YUV420P8 to YUV420P10 for x265Model
clip = core.resize.Bicubic(clip=clip, format=vs.YUV420P10, range_s="limited")
# set output frame rate to 29.97fps (progressive)
clip = core.std.AssumeFPS(clip=clip, fpsnum=30000, fpsden=1001)
# output
clip.set_output()

So you are using QTGMC and sRestore with some DVD input. (aiming for 29.97fps usually is wrong in this case, but your choice)
That the initial speed reporting is 'off' is 'okay' since filling caches etc. and often having content with less motion data at the beginning usually can irritate the speed measurement.
That said, 1.3fps seems too slow for 720x480 content on your cpu.

• Does the speed also drop if you do not use sRestore? (sRestore is a pain and sadly enforces linear processing)
  
• Does it make a difference if you enable 'Filtering->Vapoursynth->Misc->Script->Limit Cache (MB)->3/4 of system ram' ?
  
• Does it make a difference if you enable hardware decoding? (no clue whether that even works for lav or bestsource) Or try another source filter?
  

Cu Selur

Ps.: "QCoreApplication::postEvent: Unexpected null receiver" can be ignored, I fixed it locally, but it should not any any problematic side effect aside from one start up message not getting shown.