Why these files are so large
A screen recorder captures every frame of a display at full resolution. A one hour capture of a meeting or a lecture can easily be two or three gigabytes, of which the part you care about — the speech — is perhaps forty megabytes. That mismatch is why uploading one to a conversion site is so painful, and why doing it locally is such an obvious improvement.
Because the video track is discarded without ever being decoded, the size of the picture has no effect on how long this takes. A 4K capture and a 720p capture of the same length take the same time.
Trim the dead air first
Recordings like these usually start several minutes before anything happens and run on after it ends. Removing those stretches before converting cuts both the file size and the processing time, because only what you kept is encoded.