18F series performance not so good?

[tumbleweed]

Those extra instructions definitely make a difference. It's not just clock speed. If it takes more instructions to do the same thing then that's a disadvantage.

That, plus even though the memory is still banked you don't have the horrible ram arrangement that the PIC16 suffers from, and no program memory page issues.
The 18F has a hardware multiplier, dual-priority interrupts, more FSR registers.

All in all, if you have a choice between 16F and 18F, 18F is the better choice. It'll produce faster code in a lot of instances.
The only reasons I can think of to use a 16F might be price or perhaps power consumption.