Speed comes down to how apps load, and the app itself. You don't have a lot of control over that process though a more efficient site in general can help. The delay in loading will happen to anyone that uses that app, though it's more noticable on your site since you've got it front and center on the home page.
Is there any possible way of placing text over the stock slideshow slides that comes with the Supply theme?
It's totally possible though expect to have to get your hands dirty with the code, or seek external help. If you want to try and DIY I am sure the forum members can nudge you in the right direction - just expect to most of the heavy lifting yourself.