Hi Rajesh,
This behavior is listed as 'limitation and restriction' section of FWSM.
http://www.cisco.com/en/US/docs/security/fwsm/fwsm22/release/notes/fwsmrns.html#wp37777
Also here is the excerpt from the FWSM configuration guide:
'Management access to the FWSM using Telnet, SSH, or HTTPS might cause a degradation in performance depending on the commands that you execute during the session. For example, if there are 50,000 current connections and you enter the show conn command, the CPU utilization is higher than if you do not enter the command. We recommend that you avoid executing commands on the FWSM
when high network performance is critical'
As it suggests, you should prefer configuring FWSM during off peak hours