The background removal is made for photos with one main object, it is not made for pixel art or sprite sheets, unfortunately.
The best way is to use another software for that. You can do it easily with gimp, which is 100% free.
Here's how you can do it easily with gimp:
You open the image in gimp, right-click the image in the layer tree and select "Add Alpha Channel". Then you use the 'Fuzzy Select' tool (“Magic Wand”) tool to select the white background area, as well as the see through areas between the legs for example that are separate from the main background (hold shift to add the new see through area selection to the already existing main background selection), and delete it all (e.g. with ctrl + X).
As for the issues you ran into, can you give me more information on it? I would like to understand it a bit more so that other people do not run into the same issue:
- Can you export sprites without any issue and freezing?
- Does every single effect freeze? Even 'Contour White' and 'Resize Crop'?
- You are on Windows, correct? Which version of Windows?
- Does your computer device have a GPU?