CPU load issue that related to network

hello
 
i need some help with my issue here
i don't know why everytime i download big files and the speed got faster than 1.5mb (i am in japan with 100mbps connection)
the cpu http://en.wikipedia.org/wiki/Central_processing_un load jump to around 70%
thus lagging everything that running, including itunes (screeching)
is that normal ?
 
my currect system :

XPS730

intelQ9550
790iultra
9800GT
XonarD2X
WD500GB

for network settings i keep default at most
flow control http://en.wikipedia.org/wiki/Flow_control - disable

interupt moderation - enable
ip checksum offload - RX&TX enable
jumbo packet - 1514bytes (there are several option, max 9014bytes)
large send offload v1 ipv4 - enable
large send offload v2 ipv4 - enable
large send offload v2 ipv6 - enable   

low power state link speed -disable (changed)

network address -not present

priority&vlan http://en.wikipedia.org/wiki/Virtual_ - priority enable
receive side scaling - enable
speed/duplex setting - auto negotiation
TCP checksum offload ipv4 - RX&TX enable
TCP checksum offload ipv6 - RX&TX enable
UDP checksum offload ipv4 - RX&TX enable
UDP checksum offload ipv6 - RX&TX enable
vlan id - 1
wakeonlan from poweroff - disable (changed)

i wonder if monitoring system the one that cause the problem... as i am using network traffic monitor widget on desktop sidebar and G-15keyboard lcd

hope someone can help me solving this issue

thanks before