I have a very rare problem. The cart, the backend, the one step checkout takes a lot of time, more than a minute and only can be solved if I cleaning all the cache... but I do and after 30 minutes the same, other time slow. I dont know what can I do, I cant find a solution. You can test in https://www.espamoto.com
this is the network tab and it shows zopim takes quite a while to load.
this is the console tab and it shows some errors with blocked content.
It is difficult to help since we don't have logs from your server or knowledge of the modules and code you run. But my rule of thumb is always to get started with clearing what shows to be an issue in front of your eyes.. And then before you know it, you will see emerging clearer issues or better your site will be sorted.
Now, but not least, you may want to ensure your server stack is as simple as possible. For instance, disabling varnish, redis or anything that potentially can be not configured well.
--> of course, as soon as the problem is understood. you may enable these again.