The 16 years old CPU should be able to handle guitarix as long as you don't add too many modules or DSP intensive modules like NAM.

When selecting the tubes select just one tube instead the options that include both a pre and post, that will be lighter on the CPU, at very minimum you need a head a tonestack (or amp IR) and a cab, that's 3 modules, and they should run well even of a 16 years old CPU.

The latency is due to your buffer size being too high, 1024 is usually the default, you probably want to use 256 which will usually give you an usable latency without too many xruns (audio breaking up). But depends on what your system can handle, smaller buffer size = less latency = more system overhead = more likelihood of xruns.

On Gx you can set the buffer size here:

Alternatively you can use the command pw-metadata -n settings 0 clock.force-quantum 256 if you use pipewire.

You may also want to set it back to 1024 (or higher) after you finish playing so you can watch videos again without audio stuttering.

Setting the sample rate to 44100Hz I think it's also recommended, you can do it with pw-metadata -n settings 0 clock.force-rate 44100.

If you use jack instead of pipewire you can control those parameters with qjackctl (I used to know the commands for jack, but I haven't used jack in years).

However I have to warn you that you will not get good results by connecting the guitar to the microphone input, as there is an impedance missmatch, the noise isn't the only issue, the tone of the guitar will sound super muddy, unless you are handy with electronics and can make a impedance converter/preamp/DI box.

Also, I bought an super cheap 6 bucks "usb to guitar cable" from ebay, and the results were worse than the mic input, maybe I was unlucky and bought from a bad seller and there are good copies out there and YMMV, but I ended up scrapping that interface keep just the connectors. So I would rather save up for an UM2.