> Maybe I should check which part causes such difference,
> but I haven't checked any compiled code yet.

With ttcp(1) from pkgsrc on hp425t, there is few visible difference:

at peer (1.3GHz PC):
---
# ifconfig sl0 create
# slattach -h -s 38400 /dev/tty00
# ifconfig sl0 192.168.100.1 192.168.100.2
 :
# ttcp -ts -l 8192 -n 16 192.168.100.2
---

sl0 settings on hp425t:
---
# ifconfig sl0 create
# slattach -h -s 38400 /dev/ttyC0
# ifconfig sl0 192.168.100.2 192.168.100.1
---

HEAD GENERIC:
---
# ttcp -rs -l 8192 -n 16
ttcp-r: buflen=8192, nbuf=16, align=16384/0, port=5001  tcp
ttcp-r: socket
ttcp-r: accept from 192.168.100.1
ttcp-r: 131072 bytes in 41.76 real seconds = 3.06 KB/sec +++
ttcp-r: 535 I/O calls, msec/call = 79.94, calls/sec = 12.81
ttcp-r: 0.0user 0.3sys 0:41real 0% 0i+0d 0maxrss 0+1pf 534+5csw
---

gdamore-uart GENERIC:
---
# ttcp -rs -l 8192 -n 16
ttcp-r: buflen=8192, nbuf=16, align=16384/0, port=5001  tcp
ttcp-r: socket
ttcp-r: accept from 192.168.100.1
ttcp-r: 131072 bytes in 41.75 real seconds = 3.07 KB/sec +++
ttcp-r: 539 I/O calls, msec/call = 79.32, calls/sec = 12.91
ttcp-r: 0.0user 0.2sys 0:41real 0% 0i+0d 0maxrss 0+1pf 538+4csw
---

gdamore-uart GENERIC + COM_REGMAP:
---
# ttcp -rs -l 8192 -n 16
ttcp-r: buflen=8192, nbuf=16, align=16384/0, port=5001  tcp
ttcp-r: socket
ttcp-r: accept from 192.168.100.1
ttcp-r: 131072 bytes in 41.81 real seconds = 3.06 KB/sec +++
ttcp-r: 509 I/O calls, msec/call = 84.12, calls/sec = 12.17
ttcp-r: -1.9user 0.3sys 0:41real 0% 0i+0d 0maxrss 6+3pf 515+14csw
---

(Heh, 0.01 real second could cause difference of 0.01KB/sec)
---
Izumi Tsutsui