Site Suddenly Sluggish---Troubleshooting

Site URL: https://www.summitandsur.com

I had my site built out and finalized a few months ago and it was previously working fine, no loading issues that I was aware of. I went to blog today on my site and noticed that when moving from one page to another, the load time seemed much slower and lagged. So much so that the header images are showing black before they load, which wasn't happening before. 

My web designer doesn't seem intent to help so I am here looking for advice on troubleshooting. The photos on my website are already web sized and ran through JPEGMini so I *really* don't think that is the issue. The only thing I have done and/or changed in the last couple months since having this site are:

-blogged a few times

-changed the SEO title format for the home page to a different verbiage by changing the page title itself

-added a couple links to my About page

The biggest issue really is that when the pages from the main nav load, you can see a background color where the header image should be before it loads. I have reloaded my files in there and double checked their sizing---everything is correct. 

Anyone have any advice on this? I am going crazy trying to figure out where the issue lies!

Thank you!