I'd like to lay down some solutions to help you fix the slow loading issues in QuickBooks.
Are the pages or content loading slowly on all websites? You'll want to check your internet connection and see if there are any issues with it. Restart your router if needed.
However if this is only happening on QuickBooks Online webpages, then a cache-related issue might be causing them to load slower. While the cache helps webpages load faster, a simple corruption or issue can cause problems while working in QuickBooks.
One way to rule this out is to use an incognito or private browser:
Mozilla Firefox: Ctrl + Shift + P
Edge: Shift + CTRL + N .
Google Chrome: Ctrl + Shift + N
Safari: Command + Shift + N
Log in to your QuickBooks Online account as you normally would. If the webpages load properly, you'll want to go back to the regular browser and clear the cache. This helps remove the corrupted webpage or cache data that's causing the contents to load slowly.