Is it true that copying is the most CPU intensive operation?
A basic microprocessor works at a fixed speed at assembly level. For the most part it can execute "instructions" in a certain number of clock cycles and that is that. The clock is usually fixed in speed (i.e. 8MHz for example) and an instruction might take 4 clocks cycles for example. This mean it executes an instruction (in this arbitrary example) in 500nsecs.

If you programme a micro to do more complex stuff, you are usually asking it to execute a sequence of several "instructions" and this naturally takes longer - it doesn't put more load on anything (like a mechanical engineer might envisage putting load on a shaft or a beam) - it just takes more time to do more complex things.

"Copying" might mean "read a value in memory and store it in another area of memory". To do this usually involves 2 instructions; one to read the value stored in memory and a second instruction to store it in another area of memory. It can be a little more complex than this of course - you need to setup the target address in memory and you need to setup the destination address - doing this might involve a couple more instructions but does the load on the micro change? Not really because in a simple micro it would be doing something like looping and waiting for an input to change so it can process some more data.