It is possible to use permalinks to pre-load the cart which I think would solve your problem.
Dallas here from the social care team.
Unfortunately, this is going to be a pretty short message with not a lot of great news. For the buy button the only way that it's able to work is is if you actually embed it onto your site.
Even though I'm sharing bad news for this particular question I'm more than happy to give some tips that might help speed up your checkout experienc. To be honest the slow speeds might simply be to do with a build-up of cache on your browser. Have you tried clearing your cache yet? This helpful link gives instructions on how to clear the cache for all browsers. Can you try that and then let me know how it works?
If you could also try on other devices besides the one that you've been working on that would be great! That way we can gain some context of how it compares on different devices.
If you're able to take a video of your page while it loads the checkout that would be great as well so that I can see what we're working with.
I look forward to speaking with you further.