Ok, so I'm dealing with 300k records, so you won't notice the difference here.

BUT, on my db if I search for any word, all forums, 1 week, results are back in less than a second.

When I change to 2 weeks, it takes 12-20 seconds.

When I change to all posts, it takes the same amount of time as the 2 week search.

What's so special about posted newer than 1 week?

Which UBB.threads version?

An easy thing to do first is to figure out exactly what database query is used for the search, and then use EXPLAIN. E.g., EXPLAIN SELECT * FROM w3t_Posts WHERE (whatever). That will provide some information about how MySQL will do the query. Try it for a one-week query and a two-week query, and see if there's any difference in the EXPLAIN output.

Maybe your MySQL hasn't enough memory so that it needs to swap if you search over a longer period.