Speed multi part of video
Use

-filter_complex [0:v]trim=0:12,setpts=(PTS-STARTPTS)[sv];[0:a]atrim=0:12,asetpts=(PTS-STARTPTS)[sa];[0:v]trim=12:50,setpts=0.50*(PTS-STARTPTS)[v0];[0:a]atrim=12:50,asetpts=(PTS-STARTPTS),atempo=2.00[a0];[0:v]trim=50:57,setpts=(PTS-STARTPTS)[ev];[0:a]atrim=50:57,asetpts=(PTS-STARTPTS)[ea];[sv][sa][v0][a0][ev][ea]concat=n=3:v=1:a=1

concat filter expects all inputs to start from 0, so each instance of PTS should be replaced with PTS-STARTPTS.