there is comment from our expert:
1) depends on given filter response for FIR filters cpMAC (clock per one Mul-Accumulate operation) reduces with filter length while for IIR filters this characteristic is rather constant and doesnt depend on number of taps. So for rather qualitative filter FIR may be significantly faster than IIR, but you also should take into account delay for FIR and possible instability for IIR.
2) There is internal criterio that depends on filter length and data type for switching from time to frequency domain processing (usually >=16-64 taps). Freq-dom processing is used for all data types for all FIRs with pState structure and for some FIRs with direct suffix.