You're right, WEBP should have an FPS setting but it's missing. 😮 We've fixed this in 4.2.10-beta.
As a workaround, you can click Save on the export dialog to save an export settings JSON file. Edit the file with a text editor to change "fps": 20, to the desired FPS, eg "fps": 40,, then click Load on the export dialog and choose your export settings JSON file.
