On my product pages, my add to cart and payment methods load extremely slow and I can't figure out why or how to fix it. I've tried disabling extra payment methods and disabling unused 3rd part apps but that has not helped.
Here is a link to one of my product pages https://allartapparel.com/collections/mugs/products/flowing-mug
By "load extremely slow" do you mean when you click on the ATC button? Or the page loading per se?
Taking a quick look now I can see that there are tons of images on your website being loaded immediately.
On the right side you can see all the images that are being loaded at the moment the user gets on your website. There are definitely more than 10 there. However, only 5 images are visible (Logo, main image and three thumbnails)
This is a very technical issue, you'd need to get in touch with a Shopify developer to lazyload your images, compress them and subsequently improve your website speed.
There may be many other underlying issues that are taking a toll on your website's performance, this is just what I can see in a few minutes of checking.