a dummy's query, is fps drop expected from core.std.SelectEvery ?

[hydra3333]

Hello. Dummy seeking advice. In the script below, this line

video = core.std.SelectEvery(video, cycle=4, offsets=[0, 3])

in the "re-interlacing" part of script, drops fps by half, from 20fps down to 10fps, when piping to x264 and encoding as interlaced.

Taking longer elapsed, sure as there's more material to encode, but dropping the fps by half ?
I must be thinking about it wrongly, which seems likely, unless there's something very slow about that line.
Is someone able to clarify ?

import vapoursynth as vs 
# snipped imports and loadplugins
def main(): 
    core = vs.get_core(accept_lowercase=True) 
    video = core.d2v.Source(r'T:\HDTV\WDTVlive\MP4-VS\x64-1\0-test.d2v') 
    video = core.std.Trim(video,first=0,length=500) 
    # INTERLACED-AWARE DEBLOCKER IN HERE 
    video = core.std.SeparateFields(video, tff=True) 
    video = haf.Deblock_QED(video) 
    video = haf.Weave(video, tff=True)  
    video = mvs.AssumeTFF(video) 
    # QTGMC IN HERE (SLMODE=1 is spatial, SLMODE=2 is temporal) 
    video = haf.QTGMC(video, TFF=True, Preset="Very Slow", Sharpness=1.2, SLMode=2, EZKeepGrain=1.2, NoiseProcess=2)     # result is double framerate progressive, so re-interlate it later 
    # DEHALO VIDEO 
    video = core.fmtc.bitdepth(video,bits=32,fulls=False,fulld=True) #convert to 32bit float. Specify if the clip is full-range (True) or TV-range (False). fulls is for input, fulld for output.  
    #video = Vine.Dehalo(video, radius=[2, None]) 
    video = core.fmtc.bitdepth(video,bits=8,fulls=True,fulld=False) #convert to 8bit Integer. Specify if the clip is full-range (True) or TV-range (False). fulls is for input, fulld for output.  
    # RE-INTERLACE VIDEO  from double-framerate back down to normal. this is right for 576p50 to 576i25 
    video = core.std.SeparateFields(video, tff=True) 

    # if comment out the next line we get circa 20fps if we uncomment it then we drop way down to only circa 10fps ... should we expect that ?
    video = core.std.SelectEvery(video, cycle=4, offsets=[0, 3])     # looks like avisynth  

    video = haf.Weave(video, tff=True)  
    video = mvs.AssumeTFF(video) 
    video.set_output() 
    return True 
# ... functions here snipped
main()

x64 portable.