Morning
@Jim Street
The issue with Squarespace sites is like many other ready-built template sites (eg Wix, Shopify etc) they look great but come with a lot of resources that you just can't remove. For instance, your Home page is pulling 1.87MB which may seem odd given it's not particularly intensive on imagery or any particular advanced functionality. What you can't see is all the bloatware behind the scenes that is next to impossible to remove due to the very limited changes you can make in Squarespace. This makes it easy for Squarespace to manage since there is less that you can break but it also means there's less that you can fine-tune.
In fairness to Squarespace, it has one of the better load times average than many of these sites. Alex Denholm mentioned in one of your replies Brian Dean. One of the studies that he conducted (look on Backlino site for Page Speed Stats) found.
'When comparing major CMSs against one another, Squarespace and Weebly have the best overall mobile page speed performance. Wix and WordPress ranked near the bottom.'
So from this perspective things sound good with Squarespace. The difficulty is that unlike a Wordpress site can be made to perform faster than a Squarespace site by removing plugins, optimising images, getting more responsive themes and generally tinkering around with the settings this is much more limited in Squarespace.
However, given your position, I think site speed should be only a small part of your considerations from moving away from Squarespace. I can see that you aren't ranking on page one for hardly any terms that you're trying to rank for and there is some considerable scope to optimise your site regardless of whether you choose to migrate or not. I also note that you're using Adwords considerably so maybe it would be worthwhile moving your marketing budget away from PPC over to new website development or considering affordable SEO services.
PS Alex mentions the Brian Dean article about the relation to ranking and site speed (look on Backlinko for Search Engine Ranking). In it he found no correlation to site ranking and site speed. I've contacted Brain and suggested maybe further research might be in order, particularly regarding mobile site speed usage and rankings. However, even if Brian's astounding claim is true surely delivering pages to your audience as fast as possible makes for a better user experience, regardless of ranking considerations?