On the other hand, the speed of the serial port for data transfers is not really interesting at all. Lets look at the numbers:
The newest Amiga Zorro serial cards like Hypercom, VarIO, etc can reach 46KB/s, some of them can exceptionally reach 62KB/s
The vast majority of Amiga Zorro serial cards like the Vector Connection, IOBlix, etc can reach 11.5 KB/s
The low end Amiga Zorro serial cards like Multiface, GVP IOExtender, etc will never achieve anything beyond 6.1KB/s
Clockport based serial cards can reach speeds resembling those of the high end Zorro serial cards.
I know numbers are a bit dissapointing.
The parallel port is often faster, and even more in some expansion cards (500KB/s), it is in some aspects a better choice than the serial port. It can be used thru a PLIP driver for TCP-IP connections.