The MTF feature uses additional resources and therefore can slow down the speed of the backtest.
The only thing I can recommend that you try is changing the modelling quality to the control points method, it should speed up the process.
If that doesn’t help much then I’m afraid the hardware is not suitable for this and you should consider upgrading it.
Fixing the Volume on offline charts is in the to do list and should be released within the next two weeks.
Please let me know if this helps