The SQL Query would be like this...

Select CustomerID, col2, col3 from tbCustomerTbl1

where CustomerID = @parameter

If (@rowcount = 0)

Select CustomerID, col2, col3 from tbCustomerTbl2

where CustomerID = @parameter

That should retrieve the rows. Make sure you have an Index on CustomerID.

Performance increase is not an easy thing to explain. You can increase performance using a better Hardware, Query tuning and lots of small things which adds upto a big chunk of permonce increase.