I think that misunderstands the difference. FPGAs are logic devices but with registers (and memory) which means they can also perform sequential tasks. However, FPGAs can perform many tasks in parallel using logic and therefore at high speed.
A microcontroller is a sequential device and so quite slow by comparison. The peripherals of a microcontroller are implemented in hardware and so reasonably fast (eg UART) but microcontrollers cannot perform logic functions at hardware speeds. Any logic requiring any speed would need to be done outside the microcontroller.
Keith