A while ago I tested speed difference when browsing with or without HTTP(s) scanning and it was negligible. On my system pages load faster with Eset (with https enabled) than with Kaspersky, MD or Emsisoft.
If I disabled http scanning entirely it actually slowed down page loading in FF. If I only excluded FF from protocol scanning, page load improved but it was hardly noticeable.
That's my experience on my system.
