Please post the full log (Attach it, don't copy paste it). Part lines cannot help debug the isssue.
Is your priority set to Low? If so that can explain why it's running slow.
How large is your file to be converted? You had mentioned that there is some other stuff going on the computer - is that CPU or Disk intensive?
Since you're already tried reinstalling v8 with no difference I suspect it's some background process which is consuming resources and slowing down MCEBuddy. WTVtoMpeg is a VERY disk intensive process, so a loss in disk or IO performance will degrade MCEBuddy
Try to set the priority to normal or high if not reboot the computer. FYI, v10 is definitely faster than v8 on the benchmark tests.