I would know how to see whether the router is busy on defragmentin/fragmenting of packets about wrong settings of MTU. I have IPsec traffic going through the router and when it is present the CPU blasts up to 100%. Every other traffic can reach peaks of 400-500 kbytes per second while the IPsec traffic only 30/40 kBytes per second. Using "cpu show proc" the sum of all processes is close to 1% while the percentage on the top of list reaches 95/100%.
Many thanks,
Alex.