This field is a Varchar(MAX).

You probably already saw this from your exchanges with Naomi (I agree Naomi, this is a good discussion topic).

SQL Server runs through a different internal execution when dealing with VARCHAR(MAX) as opposed to VARCHAR(N). It's because SQL Server can't store/reference VARCHAR(MAX) as a straight/contiguous block of memory, since it can go up to 2 GB.

I found a link here that talks about the difference in performance being anywhere from 10% to 25%.

http://rusanu.com/2010/03/22/performance-comparison-of-varcharmax-vs-varcharn/

Maybe the other factors you mentioned (corruption) might also apply, but wanted to share this link. It doesn't offer any kind of solution if you're required to use MAX, but it does illustrate the differences in a simple test.