Trying to figure out why site loads hero/banner image slowly (even on paid Wix)

Hi Everyone,

I’m getting slow performance on the pro version of Wix, possibly due to images but there could be another culprit. It is on a domain (by itself) where I’m hosting a test site built with a free dev account.
I thought there would be a big boost when going from free to paid.

It seems to possibly be caused by the hero images, which are housed in a carousel of 5 slides. I’ve gotten their sizes down but sometimes it is taking about 8 seconds for the first image to load, then they’re all fine. They’re big but not more than 700k or so, saved as png’s. They need to be high quality due to the nature of the company, and this question is mostly for desktop. Is there a good rule-of-thumb anyone uses to make sure images are coming in at a reasonable speed?
Thank you in advance to anyone who sees this.