Submit button loader or asynchronous processing for slow button submit

Hi. I have double opt-in confirmation turned on. However, this means whenever a site visitor presses “submit” after filling out his email, the holler box hangs for about 10 seconds while this double opt in email is being generated in the background. Meanwhile, there is no visual confirmation that the submit is being processed, resulting in poor UX.

Would you have any tips on the best way to solve for this? Maybe add a loader gif through JS/CSS on button click? As far as I can tell, there is no way to speed up the email send since it’s an inherently expensive process, and PHP doesn’t support threading, so the solution seems to be to indicate to the site visitor that the request is being processed.

Thanks!