Actually, the first thing I would do to debug it would be to have the stored proc run in the actual environment in sql query editor (assuming it's not a data modification proc) and have the actual execution plan included in the query results. If it is data modification, you need to run it in a similar backup of the prod database, so you have similar table sizes and statistics. 8 out of 10 times, one hugely expensive part of the query will jump out at you. This is the quickest way to find out where the bottleneck is, because until you identify that, any 'optimizations' you do will be wasted.