My Magento store is slow but only during the first request when it isn't accessed for a while. Every request from any PC or IP after that is blazing fast and I can't figure out which type of cache is causing this.
The server itself is very simple, just apache with php-fpm and OPCache. No varnish, solr, redis or anytype of other cache. The store itself is in production mode with all cache enable and including full page cache. Static content is properly deployed and DI is also compiled properly.
I have a feeling that this might be caused by OPCache unloading the cached PHP files after a while. Does this sound plausible? Is there any way to fix this issue?