>I'm not sure how the EnableEventValidation setting would have any effect on a 'Invalid viewstate' error.
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>Setting EnableViewState to false would certainly stop to error since there would be no ViewState to validate <s>. But whilst you may not need to maintain ViewState on this particular page I assume there are other pages where this is not so - and those pages are probably still prone to the invalid viewstate error.
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>I'd suggest implementing the fixed key as recommended in the link Tracy supplied.

You are right. I immediately found out that only enableEventValidation was needed for me in Web.config as the other one should remain to its default. Otherwise, the state would not be maintained. So, I adjusted it back after. But, it is surprisingly interesting to see the difference in speed for the few minutes that I didn't have it. It would be nice to be able to get that speed and preserve that setting. I wonder if Microsoft could improve that algorythm in a future version so performance will be much better.

As for the invalid viewstate error, I will have to go with the fixed key as you both mentioned.