To give you better visibility into the sync process with Marketo, Knak has created a new feature called Knak background sync.
To enabled this in your account contact Knak support or your customer success manager.
Once enabled you will see a new screen when you try to sync an email showing you the sync progress. If this is taking a while you can close this window and keep on working.
To check the progress of your sync click on "Email Sync' in the left nav menu.
Here you can see all your synced emails including any failures and the reasons for the failure.
This should give you a better idea how long emails syncing is taking and any issues.
Need more help? Contact support via live chat within Knak using the chat bubble in the bottom right corner or email firstname.lastname@example.org