Why is Reporting Services report much slower than requesting it?

Question

I have a request that takes about 2 minutes. It is not very difficult in terms of parameters or something else, and the report itself does not do really extensive processing. Basically just spinning the data right in a good format. (In fact, one of the reports does not format the data at all, it simply returns a flat table intended for management in excel.)

It does not return a massive dataset.

However, the report takes 30 minutes to run.

What could be the reason for this?

This is SSRS 2005 vs. SQL 2005 Database.

EDIT: OK, I found that with WITH (NOLOCK) added to the report, it takes the same time as the request through SSMS. Why is the request handled differently if it comes from Reporting Services (or visual studio on my local computer) than if it comes from SSMS on my local computer? I saw a request executed in Activity Monitor a couple of times in SLEEP_WAIT mode, but nothing was blocked ...

EDIT2: Connection string:

Data Source = SERVERNAME; Start Directory = DataName

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