I’ve had a problem the last month or so (and now today). Besides the ‘slow’ loading of the website 85% of the time, I’m starting to get a wordpress error that some plugins failed to load. After logging into recovery mode, I check the plugins and sure enough, there are some php files missing from the folder. Have to delete and reinstall plugin.
WordPress, especially when loaded with various plugins, is a complex beast with a lot of moving parts.
Looking at your site, the first request is indeed VERY slow but everything after that is quite fast. This is generally caused by background tasks running in WordPress. When a site has been idle for a while, WordPress executes a lot of maintenance and background tasks on the first page view. Plugins can also load additional tasks to be executed.