Does adding a machinekey in the system.web of your web.config cause any performance problems?
I added something like this to my web.config:
<system.web>
<machineKey validationKey="aaa"
decryptionKey="bbb" validation="SHA1" />
</system.web>
and now it seems to run quite a bit slower. I am using firebug to see how fast the page is loading.
I ended up with this problem after this issue
Note: this is not the actual machine key I am using. I have a proper machine key.