SQL Server 2008 R2 to MongoDB - is it safe to migrate?
What kind of issues do you face with SQL Server 2008 R2??
One option might be to figure those out and fix them. If you optimize your stored procs, if you redesign your table and indices to be more efficient, that might be more than enough to keep your app up and running very well.
I don't know MongoDB that well - but the basic approach is very different - instead of rows and columns and relations, you have "documents". As far as I know, MongoDB does not have any construct similar to stored procedures or triggers - all that would have to be handled in your application.
So it really depends on what kind of app you have - something like an accounting application is probably better off in SQL Server 2008 R2, while something else might be more suitable for MongoDB.
I don't think switching to MongoDB is a "quick fix" just for any kind of performance problems...
Also check out this other SO post on the very same topic: