reason for slow

i have just reinstalled my server from FC2 -> CentOS 4.2 and kernel 2.6.5-1.358smp -> 2.6.9-22.0.2.ELsmp
however i found that the server is running much slower than before, estimated from 2 - 5 times (the difference compares with that before reinstallation is obvious)

is there any way to figure out the reason ??

 

 

 

 

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