[Lizard]

Yes, it can take several minutes for a task to make it through the whole system.
Siri's Reminder needs to sync to iCloud.
Then OmniFocus iPhone needs to sync to iCloud.
Then OmniFocus iPhone needs to sync to Omni Sync Server (or whatever WebDAV you're using).
Then OmniFocus on Mac (or iPad) needs to sync to Omni Sync Server (or whatever WebDAV).

These syncs are mostly independent and all run on different timers. So it can take a while. On the other hand, the iPhone is what you used to create the action, so you're perhaps most likely to check your iPhone OmniFocus next, and that transfer only requires the first 2 syncs.