The short answer is to minimize the number of tabs open or try to use another browser. Edge has worked for me in some scenarios, even though I hate using it.
Make sure that you have caching turned on. Open the browser's developer tools (usually F12) and uncheck "Disable cache" (Chrome). Mine is checked, so this should be UNCHECKED.
Turn off any throttling on your web browser.
If you're asking for any real reasons why it may be slow, here are a few things I found
1. There is analytics that is referenced every time a page is loaded. From what I can see, that's google, aws, and some yahoo. It could be that this site is hosted on one of those cloud services.
3. Files you download could be consolidated into fewer files, like vbulletin's editing library. These files could also be loaded asynchronously for a better experience.
4. External files. Something simple like an icon is hosted on another site. If those files are unavailable, then sometimes the request has to be timed out which can take a while.
Even with just your post, it takes about half a second just to load the page content. The other colors are other resources used by the page which adds up quickly.