There is a smaller ID on PEX compared to CPVC (0.485" to 0.546" for 1/2" pipe), but the connection system also affects flow rate. If PEX is crimped on the fittings, there is a further reduction of flow. Expanding the PEX on F1960 fittings eliminates that restriction at least.
Most people don't see a big flow rate drop with expansion (F1960) PEX compared to copper or CPVC. You can always upsize the tubing if flow a concern, but the downside is that hot water takes longer to reach fixtures with larger ID pipe.
John