I like what you did on the document details table.

Did you also create a nonclustered index on DocumentCodeList(invoiceid, srnumber)?

Of course, the obligatory question is if you understand the dangers of using NOLOCK.

The other obvious question is about the three tables involved. Please tell me that they have clustered indexes on them.

From there, take a look at the actual execution plan and find your bottlenecks. If you don't see it, please post the DDL for the three tables and the actual execution plan.