And secondly, I saw something in the database while looking for a solution. I don't know much about asp.net, but I do know something about SQL, and I saw that the stored procedure "EventsGetByRange" is built up using a string variable.

One thing I remember from being a database administrator (years ago) is that it's never wise to build up complicated queries using varchar variables. Normally, when running a sql or stored procedure, sql server caches & optimizes the query plan according to the indeces and what sql server thinks is the fastest approach to get the result. Maybe this changed, but as far as I know this doesn't happen when you build up a query using a varchar string, then, sql server can't "analyze" the query.