Hello Migingoh, thank for reaching out about invoicing speeds, the best thing to do would be to check that the system requirements for QuickBooks are being met. Here is a handy article to look at what to check. If you do not meet the requirements, please considering updating your operating system so that you can continue using QuickBooks.
If you do meet the requirements, can you try clearing the cache and cookies on the browser, selecting all time in the date range in case that is slowing things down.