Yep, the only thing faster than _MEM are BITWISE operators, the reason for this is the same as _MEM being faster than normal variable interactions. BITWISE operators are processed 'in house' within the processor, they never have to travel on the BUS to memory and back. they stay in the CPU, so you cut travel time nearly completely out. With today's BUS speeds the difference is getting smaller though. And when your working with variables the standard way there is a lot of back and forth between the CPU and RAM before the result is done.
You could think of it as needing a vegetable to eat,your property seen as the CPU, is faster to walk out to your garden and back than it is to take the bullet train to the store, seen as RAM, and back. its all a matter of distance traveled, and even at the speed of light(or 99% of it as electrons travel) it still takes time. As the standard use would be like Calling the store to make sure they had the vegetable, then calling a Cab, waiting on the Cab, going to the store buying the vegetable, then calling the Cab again, waiting on the Cab then getting home with the vegetable. the more travel and talking you can cut out the faster things happen.
_MEM gives you a heck of a lot more options though, as BITWISE operations are very limited being restricted to a single variable. Where as _MEM can act on a large group of variables. but that is getting into Apples vs Oranges territory there.
Granted a rather Simplified explanation, and may be a bit short here and there.