I think you'll find that Discord is lightweight, relative to the majority of users.
Building for low spec machines has an incumbent cost upon it. Performance optimization is challenging.
Many projects I know build for the "middle quartiles," which means that if you're in the bottom 25% of machine specs nobody bothers to test/optimize for you.
> A Discord community that I moderate
> The influx of large numbers of people also tends to lower the quality of discussion and before you know it, you're an unpaid warden who ends up annoying a few members of your community who end up forming hostile communities and raid your servers.
This is internet moderation in a nutshell. If you're surprised and disappointed at this, administering internet communities probably isn't for you.