If it is only slow during online banking my guess is you have saved every transaction that has ever been downloaded.  Try this:

Go into QuickBooks->Preferences (it is called QuickBooks->Settings on Ventura as Apple renames the menu).  Click on Data Entry.  Click on the Reset button for One-Time Messages.

Then go to perform online banking.  When the Download transaction Window comes up go ahead and process one transaction (adding it to the register or matching it, whatever).  Then close the Download Transaction Window. 

You should see a dialog asking your to delete the downloaded transactions.  It may be worded the opposite way ...  I can't bring it up right now - but whatever it says make _sure_ you say "Yes delete the already matched downloaded transactions".

Did that fix the issue?