I'll share the steps on how to make your QuickBooks Online (QBO) run quickly. Since QBO is browser-based accounting software, you'll want to start the troubleshooting steps by access your account using a private browser or an incognito window. This mode doesn't use the existing cache data and helps us confirm browser-related issues. Here's how:
For Google Chrome browser: Ctrl + Shift + N
For Mozilla Firefox browser: Ctrl + Shift + P
For Safari browser: Command + Option + P
After that, try saving an invoice again. If it works, go back to your regular browser and clear the cache to delete those temporarily stored files and browsing history. The overtime collection of data can create corruption, however, removing this should fix the issue.