What version are you using? 3.7 has much better GIF exports than previous versions. There are many new settings and the GIF quality is as good as the GIF format allows.
Note the Spine GIF export's goal is not to make the smallest GIF file size, instead the priority is to make the highest quality GIF and a small file size is secondary to that. You can use other tools to reduce the file size of the GIF that Spine exports, but the quality will suffer (sometimes severely).
Unless you are exporting extremely high resolutions, there should not be any memory or heap size problems. Spine also doesn't require a powerful computer. Cropping and scaling can reduce the resolution, so GIF export will be faster and use less memory. What is the resolution of a GIF that fails to export?
FPS controls how many frames are output, so of course has a big impact on the export speed and file size.
Quality of 10 or 9 can make GIF export very slow. 8 is a reasonable trade off for speed vs quality. Compared to 10, a quality of 3 is 5% lower quality but about 8x faster. Quality of 2 or 1 turns off more features and the file size can be a bit larger, but export is as fast as possible.
The number of colors mostly reduces file size but export speed is also a bit faster with fewer colors.
Your skeletons, how many meshes they use, etc doesn't really affect export speed.
Spine should never crash. Please post your spine.log file (the forum will redact it) after a crash:
Windows: <user home folder>\Spine\spine.log
Mac: <user home folder>/Library/Application Support/Spine/spine.log
Linux: <user home folder>/.spine/spine.log
