Changing Database, which is the best route?

Ok, if I said that, it is because you are running SQL Compact Edition (CE), and yes, it is slow. You can check this by double-clicking the SambaPOS title in the top-left. This will reveal the Title bar, and the last 2 characters indicate the Type of DB you are running. If it says CE, you should definitely upgrade your engine.

You need to download and install SQL Express 2014:

http://www.microsoft.com/en-us/download/details.aspx?id=42299

I recommend the full package for x64: ExpressAdv 64BIT\SQLEXPRADV_x64_ENU.exe

Follow these instructions to install and configure SQL Express:

Once you have it installed and configured, you need to convert from CE to SQL: