idol for a long time before execute cmd

my server is Debian etch (stable)

2 or 3 days after server reboot
execute cmd will idol for a long time

example:
=======in my server================
someone@myserver:~$g++ prem.cpp
(idol more than 40 seconds)

someone@myserver:~$
(compile ok)
(execute the cmd second time)
someone@myserver:~$g++ prem.cpp
(less than 2 second)

someone@myserver:~$
(compile ok)

in other debian server or
after my server reboot:
===========================================
someone@myserver:~$g++ prem.cpp
(less than 2 second)
someone@myserver:~$
(compile ok)

while the idol time , the g++ prem.cpp in ps -aux
is S+ stats ?
cpu useage is 0 %

what happend is it ?
the harddisk in active
and cat /proc/meminfo

MemTotal:       508240 kB
MemFree:        257996 kB
Buffers:         83000 kB
Cached:          90588 kB
SwapCached:          0 kB
Active:         109528 kB
Inactive:       111960 kB
HighTotal:           0 kB
HighFree:            0 kB
LowTotal:       508240 kB
LowFree:        257996 kB
SwapTotal:     1052248 kB
SwapFree:      1052248 kB
Dirty:              92 kB
Writeback:           0 kB
AnonPages:       47916 kB
Mapped:          15216 kB
Slab:            24640 kB
PageTables:        684 kB
NFS_Unstable:        0 kB
Bounce:              0 kB
CommitLimit:   1306368 kB
Committed_AS:   218436 kB
VmallocTotal:   516088 kB
VmallocUsed:      3092 kB
VmallocChunk:   512504 kB

what happend is my server ?
why my server idol for a long time for execute the cmd first time?

any information need for server
just tell me , and i will post the information here