Large SQL Server advice
2GB of RAM is barely acceptable to run Windows, let alone SQL Server on top.

Obviously your 15GB of data doesn't fit into 2GB of RAM. So, any and every query you run that requires rows that aren't in RAM will require reads from disk.

From there, you can do the math on how much data you're requesting versus how fast your disks can deliver it. Given that the server is so woefully underspecified for memory, one can assume you've got just a single spinner delivering IOPS at miserably slow rate, perhaps 50MB/sec for a sequential read but as low as 1MB/sec with several queries fighting for their share.

Have 5 users each run a query that needs 1GB of data that isn't currently in RAM and you'll be waiting 15+ minutes for the data to crunch its way through the system.

RAM is comically cheap, especially compared to the cost of you wasting your time trying to optimise 15GB into 2GB. Fill the server, or buy a new one.

Also: the Pentium E6300 seems to be a desktop-class processor. You want a computer designed to be a server. Just adding a couple of disks and more RAM may not be satisfactory.