I am using the MK3/4 with the old USBCNC soft/firmware (not TNG). But I have experience similar jerkiness, slowness and sometimes lost steps when activating the path blend option. It may have to do with a combination of other settings but I could never figure that out. When "path blend" was deactivated, the problems went away.
So, I am not sure if that may also apply to the TNG software but it is something you could try.