The observation from @ahaw021 seems like it might have impacted things too. Maybe something about the switch to HTTPS caused more page resources to be served directly from your site (which was configured to significantly limit download speeds) instead of indirectly via the CDN (which would have given people a fast download). So, now that you’ve fixed up your origin server, you might also want to check which resources are now coming from the CDN and whether you’re getting the speed benefits from the CDN that you expect.
I’m glad that HTTPS itself is not responsible for the performance problems and that they’ve been resolved successfully.
I’ve had a similar experience where web server issues started right after a certificate renewal. In our case someone had modified the Apache config file at an earlier time–but did not have Apache reload the config. When the cert renewal ran, the config files were reloaded and the problematic config file changes caused a server failure.
Running your site through WebPageTest, looks like changing your site over to using HTTP2 will give you a significant speed boost.
Time to serve HTML asset on your site could use a bit of TLC.
Since you’re running WordPress the procedure for speeding up your site is fairly well known + straight forward. WPT contains many write ups about how to speed up serving your initial HTML asset.