Signal desktop runs as multiple processes. Did you add up all their memory consumption? On my Debian laptop, Signal processes consume about 700 MB of RSS in total, immediately after starting.

There's no secret really, just don't import junk dependencies like most people do and spend some time inspecting your memory usage every now and then, with very little effort you'll probably cut down on your memory usage significantly by doing that. With a lot more effort often you can probably even make something faster than uses less memory than a supposedly native app (if they spent less time than you optimizing it).