I think there is something else going on here. You cannot even add ping or ICMP traffic to a decryption policy and pings will not be decrypted and should not be impacted/impacting the load like that. I have a few 220s in the lab and have not seen this behavior. Might be time to look at logs and pcaps when you are seeing this behavior.
As @hshawn mentioned the issue wouldn't be caused by SSL Decryption, but it certaintly could be putting enough of a load on your firewall that the issue is caused because of SSL Decryption. I would really look at the overall device health when you are enabling SSL-Decryption, such as SPS count and the like and see if your box isn't hitting some other limit other than CPU.
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