Sven Fechner at Simplicity Bliss recently published an article on how to speed up OmniFocus synchronisation with great tips to get a significantly faster sync time.
A few years ago, I too noticed that synchronising between iOS devices and Macs took a long time. It happened back when I was syncing via MobileMe, which was notoriously slow in general, so I just lived with it for some time assuming that was just the way it was. But when I switched to the OmniSync Server I didn't see the increase in speed I was expecting, so I began searching for information and testing ideas on how to get faster sync times.
After a lot of trial and error, I came to almost the same conclusion as Sven. His advice is good and certainly worth following.
However, I did one additional thing: I deleted unnecessary attachments.
Attachments can be big, and if they're truly attached (as opposed to just a link) in OmniFocus, they'll sync over to your iOS devices. The image above is of a quick test to show how an attached image in a task shows up in OmniFocus for iPhone. That png is 85 kb.
One common way to end up with a bunch unnecessary attachments is by creating tasks from emails. I do this often and every single image in an email gets attached to the task. That includes useless things like logos, buttons in newsletters, and other graphical elements. It all adds up and there's no reason to have them there.
How to delete unnecessary attachments in OmniFocus
Deleting attachments in OmniFocus is easy, but you have to do it from the Mac version. Just go to Window > Attachment List.
The Attachment List window opens up. Here you can see all the attachments in your OmniFocus library that will sync over to other devices. Just select any attachment you don't need and click the delete button in the lower left.
Just be careful that you don't delete the ones called "icon.png" though. Those are your custom icons used for Perspectives.
The first time I did this, my OmniFocus database wentfrom over 8 MB to just over 1.4 MB and sync speed was drastically faster.
I now have a recurring task in OmniFocus to remind me to do it called OmniFocus Clean Up which includes archiving old data, deleting unnecessary attachments, and deluting old backups. And it's due today, so I better get on to it.