Can you upload one conversion log file?
2.3.15 should be faster than set versions but again if you've set the priority to low and are runnin other background tasks depending upon how much CPU time is available the conversion can take a long time or short time.
If please ensure your power adapter is of the right rating. Something I ran into recently. If the power adapter is under rated the bios puts the CPU into throttle mode which cuts the performance by 75% and was severely limiting my available CPU power.
Unfortunately the only way to tell this is to run a program like CPUZ and see the CPU speed and it never reaches the rated speed (not utilization) it's being throttled.
Also with windows , if you are in power saver profile - windows limits the CPU to 50% speed capacity no matter what you max CPU utilization is set to. Please note CPU utilization and speed are different. In power saver profile Speed is reduced (even when
plugged in) so CPU utilization will increase for the same tasks. Ensure you are NOT set to power saver profile even if plugged in.