First you need to be sure that your FWSM is configured
to do DNS packet inspection. Try to find out with one of the following commands:

show run | i dns
show run | b policy
show run | i inspect

If it is there, that it, move along to the next step. If not, you have to configure it:

conf t
policy-map "your_policy_map_name"
 class inspection_default
	inspect dns maximum-length 512

To check the CPU utilization on FWSM:

show cpu usage


For the IPerf command to stress the FWSM's CPU with a lot of traffic I recommend the following options (iperf.exe under Windows, iperf under Linux / Mac):

iperf.exe -c "ip_address" -p 53 -u -t 10 -i 1 -b 1000M -f m -l 1000

Below you can see the options that I used for IPerf:
-c = client (DNS server)
-p = port (53 DNS)
-u = UDP transport
-t = time in seconds (10 sec)
-i = refresh time in seconds (1 sec)
-b = bandwidth (MBytes)
-f = format to report for bandwitdh (Mbits)
-l = buffer length to write / read 1000KBytes

IPerf support many more options. You can find them by typing iperf.exe -h under Windows or iperf -h under Linux / Mac.