Indexing is something that you have to do carefuly. Study the Data and first try to find out what is the main criterea in almost all the queries. If it is based on CustomerID , I would say add a Clustered Index on CustomerID. That should speed up the query a bit.

You could try replicating the DB and run all Select queries on the replicated DB and the updates and insert on the Main DB.

If Data Integrity is not so important you can write queries with hints.

For example

Select Col1, Col2, col3 from tbCustomerTbl1 with (readuncommitted)

where CustomerID = @parameter

This will be a bit more faster than without Hint because in this case, it does not have to aquire any kind of lock. But you might select data that has not been committed yet. (maybe this non committed Data might get rolled back)