It sounds like you have a Core i7-9xx machine (quad core, triple memory channels) running a 32-bit OS (maximum addressable memory of around 3 GB after space is taken out for hardware). To get the system to use all of your resources you'll need a 64 bit OS and a 64-bit build of GIMP. 25% would be using one of your cores, which means that it's quite possibly not a multi-threaded build for the operations you're using.
All speculation on my part, of course.