Hi Fernando,
One useful technique is to look at what is happening within your database beyond just user load. If you are interested, Cotega has a (free) utility that allows you to
analyze your database for things like:
Top 10 Queries by CPU Time
Top 10 Queries Using the Most Cumulative CPU Time
Current Connection Information
Top 10 Blocking Queries Having the Longest Total Elapsed Time
Row Count Aggregate Information (total rows, minimum rows, maximum rows and last rows) for queries
Top Expensive Queries by Logical Reads
Top Expensive Queries by Logical Writes
Top Expensive Queries by Worker Time
Quite often by looking at the results of these areas it can help get more specifics on what might be causing the slowness.
I hope that helps.