I now have my first bigger project of my own. I invested around 500 working hours and the project is done. It's actual ASP.Net with C# and Linq to SQL, the platform is an MS SQL Server 2005 Express edition on a Windows server 2008 Web edition - which costs me 40€ / month (free sql and server 40€)
After going live, the website has around 20 to 50 simultaneous users.
The site is slowing down more and more, I spent the last 20 work hours to make the database queries more performant but only to get site load times from 10s down to 9,0s.
I really invested a lot of time in diagnosis and it looks like it is the database / DBMS that is slowing everything down!
I know about the specific limitations of the SQL Server Express edition - e.g. 1GB of RAM, but in my case CPU and memory usage by the SQL server should be minimal: 5-10 queries per user per minute x 20 users = 100-200 queries per minute.
In case I need another DBMS: is there a equivalent to MS SQL server that is free and without limits? (i dont think MySQL is possible with Linq to SQL on a allready finished project atm). As this is for a non-profit organization, I unfortunately do not have the money to spend even for the Standard edition of SQL Server.
UPDATE 1
Details:
Server: AMD Opteron™ 1381, Ram: 4GB, 64Bit,
Site is: http://www.rpone.de/FeatureList.aspx
The application uses 7 different queries which are rather simple like get a user from user table where userID == userid and such things, nothing special at all.
The schema is very normal: I have all PKs as Guid, all Tables have FKs.
I will be glad to provide more detailed information if needed.