if i was to guess, i would say the convertor is using all 4 cores to the max, so everything else seems slow.
if you want to still use the pc while doing the conversion, u could change the priority of the convertor program
find the task in Task Manager, then set the priority to below normal, then it'll just use the spare processing power, and not all of it.
it will slow down the conversion abit, depending on what ur doing on the pc