This is not the normal behaviour of WordPress.
As others have mentioned it could be JS render blocking as the Database has already finished grabbing the data and displaying.
I would first investigate your browser console and see what its returning on the network tab and if the console has any 404s as a broken font or a broken image can cause delays.
Also if you have any images on the posts, check their size, if they are over 1MB in file size and above 1920 x 1200 in resolution then you are causing slow downs as the files are big. (Videos too, always, always use YT or Vimeo, anything that is not stored on your local server, it will slow it down unless you are on a high end server)
Finally, if possible run the site through https://gtmetrix.com/ as it can help identify where things are falling down and slowing the site down.
PS,
You mentioned you edited the_content function, if you edit the WordPress Core, this will be overridden when WordPress upgrades. Never ever edit the core of wordpress or a plugin.
I hope this helps in some way! If you have any queries just fire away.
Cheers