ssd slow (in Linux)

If your SSD has become slow, it could depend from the swap file: f.e. in fstab the ssd swap UUID could be wrong (after some partitions changes in your SSD).

You can fix this by giving to the swap partition the correct UUID.

avidemux too slow

It’s better use (one of) the following codecs for encoding

  • x264
  • libavcodec MPEG-1/MPEG-2
  • libavcodec MPEG-4
  • Xvid 

so Avidemux will be able to optizmize the CPU usage and will be much faster. And the CPU temperature will be lower.
Cfr.here.