Why CPU utilization of a process 'ksoftirqd/X' increases when iptables are enabled on the system ?

Issue

• Below iptable rule is added which will drop INVALID packets.

[root@xxxx ~]# iptables -L INPUT -n -v | grep INVALID
  58M   41G DROP       all  --  *      *       0.0.0.0/0            0.0.0.0/0           state INVALID

• When iptables are enabled with above rule in place, the CPU utilization of process ksoftirqd/X increases to 80-90% .
• When iptables are disable the cpu usage of ksoftirqd/X is 13-18%.
• When malformed packets are send, they get drop by above iptable rule and CPU utilization of ksoftirqd/X process increases.
• Why iptable rule when enabled, causes CPU usage rise ?

top - 02:17:29 up 2 days, 16:12,  2 users,  load average: 1.30, 1.50, 1.07
Tasks: 233 total,   2 running, 231 sleeping,   0 stopped,   0 zombie
Cpu(s):  7.9%us,  4.9%sy,  0.0%ni, 84.9%id,  0.6%wa,  0.2%hi,  1.5%si,  0.0%st
Mem:   3926168k total,  3645548k used,   280620k free,     8704k buffers
Swap:  2064280k total,  1790088k used,   274192k free,   197364k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND                                                                                                                           
    9 root      20   0     0    0    0 R 94.2  0.0  66:13.07 [ksoftirqd/1]                                                                                                                     
15989 ccmservi  20   0  340m  28m 9.8m S  5.8  0.7  65:16.60 /usr/local/cm/bin/RisDC