So one or more of the valves were sticking before the engine warmed up completely, if they were sticking closed that could indicate a failed or lazy lifter, too tight valve guide or misaligned guide, or a burr. If sticking open that points to too tight vale guide or misaligned guide or a burr. That slap you heard could have been the piston hitting the valve and closing it.
If the mechanic over reamed the guides the valves could be binding because of too much slop or misalignment, if he knurled the guides they could just be too tight because he didn't ream enough and as the engine warms up they expand and allow easier movement, it's called morning sickness.
Now it's drinking oil, anyway I look at it you still have to pull the head, examine and measure those guides.