Big fan, as a very active GitHub user the conversion to GIF for screen grabs had been a constant paper-cut
[0] https://www.cockos.com/licecap/ [1] https://www.cockos.com/licecap/licecap131.dmg
ffmpeg can work videos down much better
sharex only provides a MITM utility for ffmpeg, any issue is likely to come from either GDIGrab, or the linked screen-capture-recorder utilites (which are provided within sharex as an optional download)