In a nutshell:
The PIT is a simple periodic timer. Similar to the SysTick (which is part of the ARM core).
The FTM is a more flexible timer (f == flex) which has multiple channels, input capture, output compare, combined modes, quadrature sampling, ...
The LPTMR is a special timer which can run in low power mode and is able to wake up the system in most modes (if configured as such.
About performance: this all depends on the clock input settings, but I would say they are the same.
Prescaler: this slows down the counting speed of the timer, so useful to keep the timer running at a lower speed. Of course if you have the timer configured to generate interrupts, the interrupts will happe less frequent too. The CPU performance is mostly depending on the core clock speed, and depends what is important for you: if you have too many interupts, of course your CPU won't be able to do much outside of interrupts, if all your time is spent inside the ISR.
I hope this helps,
Erich
