This in a nutshell is why any device like this will be obsolete in a few years. Why you will need either an IT background, or know a IT person to keep these units running long term.
You purchased a cheap computer, in a nice case, applied updates. Then your library has grown over time. This sounds like an indexing issue.
Any 1gb network will have 0 issue streaming music, you need around 5mb to stream hi-rez. Unless your network is not setup properly, it's not your network.
CPU intensive? To stream music? Nope, it's not, it's disc intensive, buffer/ram Doing DAC is CPU intensive, but just the TCP/IP stream? Nope
Back to the disk, think if you have a big collection of music, you need a fast disk with good cashing, and error correction buffers. This will cost 2-3x over a cheap disk.
Chances are the software has indexing jobs that run on some time table. These can make anything come to a stop. Yes, having more resources will help, but it is an entire system process.
Yeah, having a shell open, seeing everything that is running, what is taking up resources. https://www.unixtutorial.org/commands/top/
Honestly, the easiest way to stream your cataloged music, if to have it on a high quality USB disk, plug that into your DAC or streamer. Keep the music separate from your "computer" . This way you can upgrade the chain and not affect all other parts. You separate the OS from the data.
Or you get a NAS, learn about networking and QOS, mount that drive on your endpoint. My NAS failed (network card failed), just moved everything to an external disk. Simple, is always better.