How to make SSRS 2005 run fast in general

If we're talking SQL Server 2005, the SQL Server Reporting Service is a scheduling engine. It does not determine the web application side. So changing the service account there shouldn't have an effect on the web app. With that said, how are you changing the service account? It should be done using Reporting Services configuration and SQL Server Configuration Manager.

You didn't indicate what version of SQL Server. Keep in mind SSRS 2005 requires IIS installed on the box as well. SSRS 2008 does not. It uses HTTP.SYS natively. The optimal answer is #3, however, it really depends on the existing load on the physical server as well as the type of reports you would be running. #1/#2 are essentially the same option with respect to performance.

The truth is, without knowing the performance characteristics, no one here can tell you the best option. Your best path is to get a good idea of how your SQL Servers are performing today, to set up a small dev SSRS environment and profile it as you test the reports, and then make the decision for there. If you're not sure where to start on getting a performance profile on your SQL Servers, allow me to recommend:

Brent Ozar's posts on Performance Tuning