Attain 3.10 Migrate to SQL Server 2000

Attain 3.10 - I have done a migration from Navision Application Server to SQL Server 2000.
We have a 150 user site and running on SQL Server has created some serious performance problems. Too many table locks, reports take forever to run and system generally very slow.
Can somebody please give me some advise on the following:
1. SIFT Tables - for every key in the base table, we have a SIFT table in SQL. Example - Item Ledger Entry table we have 20 keys, we now have 20 additional SIFT tables in SQL and I would imagine that every one of these tables get updated every time you insert/modify a record. The base table has 3,5m records. Just one of these SIFT table has 4,7M records. Do we really need these SIFT tables in SQL - what happens if we set the MaintainSiftIndex property to NO for some or all keys? Will this have an impact anywhere? Will it improve performance?
2. Locking - who controls locking? SQL or Navision? What is the most efficient method of locking?
3. Any other ideas on making the system run faster?

Any advise will be appreciated.