There were some outages with Shopify today. If this isn't usual behavior that could be it. You can check Shopify status here: https://www.shopifystatus.com/ (looks like they're all good now).
If it's a regular issue, here's the approach I'd take to fix:
1) Make sure you don't have console errors. I see 2 related to zendesk here:
To find and fix console errors, usually you'll need a developer but you might be able to solve, here's a guide I wrote to help: https://speedboostr.com/finding-and-fixing-server-errors/.
You can also implement resource hints / preloading so that inter-site navigation is faster. Here's how you do that: https://speedboostr.com/resource-hints.
You could also start with standard Shopify optimizations. Use the Shopify Analyzer to see what can / should be optimized on your site + links to guides you can use to fix them. That's a free tool my team created for the Shopify community.