The performance of the slirp interface has improved however it's still very slow compared to the TAP interface.
On my machine:
TAP32: localhost <-> colinux approx 6..10MBytes/s
This is reasonable but not really anywhere near the disk limit, processor usage is high but there appears to be some headroom.
SLIRP: To Colinux 3MByte/s, this is OK but isn't limited by anything obvious, neither the CPU nor the disk are maxed out.
Slirp, from CoLinux 500Kbyte/s
This is so bad that I had to add a TAP interface for X. Everything looks idle; the 100Mbyte file tested was cached in RAM.