If you’ve ever used the Lens Blur effect in AE, you’ll know that it’s an unwieldy beast at the best of times, and tends to be one of those things you add right at the end (or leave switched off until it’s render time).
It doesn’t help that it’s one of the few effects that’s not multi-processor enabled, which can be deeply frustrating when you know that your system isn’t being used to its full capacity.

So here’s a quick tip on how to replace the lens blur effect with a cheap trick that employs a feathered mask and the fast blur effect (which is MP-enabled). It’s not massively sophisticated, but if you’re just looking for a quick selective focus effect, it’s a LOT faster to produce.