Buy SSD!
I have never had a problem with a folder action not firing off! Unless I have like 3-4 levels with folder actions or more. As a matter of fact, I have had folder actions to keep Finder windows neatly organized!
I don’t know why your hazel script doesn’t work, I’d rather address that with the guys behind Hazel.
An alternative solution, would be to have Hazel call a shell script, that contains an OSA script, that does the stuff with OmniFocus.
I must say, that I am opposing the usage of Hazel to accomplish this, this is something that Apple should have made working properly by their folder actions.
You can find the process by peeking into launchd, and reading up, then you can renice the Folder actions process so that it runs with a higher priority.
Apple really gives it all to the GUI, so you won’t really notice any lag if you change the priority of the launchd process that runs the folder actions for the better.
There are some reading involved, and no guarantees.
Edit:
I am sure Hazel is a great program, and the ability to sort things out with rules, like mail, is just marvellous. But having to use Hazel, in order to make something simple, that could have been done with a simple Folder Action, is clearly overkill.
When I ran many Folder Actions I reniced the process, and it worked, without any feeling of lagging. But I do have an SSD disk. Another way to circumvent this, is to make a ram disk, and install the folder action onto the ram disk whenever you boot, That should also help a little for the responsiveness of the Folder Action.