In older versions of Emagic's Logic Audio, there was an audible latent delay when using PowerCore. Emagic now supports DSP cards within Logic Audio with version 4.7.3, which includes a latency compensation option. We've tested this option with the Powercore here, and it worked very well.
To enable this option in Logic, go to the AUDIO menu, and choose "Audio Preferences". The option "Plug-in Delay Compensation" will appear toward the bottom of the window. Click the box next to the option to enable it.


Using Powercore plug-ins will still use a small amount of CPU power. This is because Logic needs to "prepare" the audio to be interpreted and processed by the PowerCore. Then Logic has to re-interpret that information once the PowerCore processs the audio and send it back. All this takes a small amount of CPU power.
Our tests on a G4/733 showed that about 10 PowerCore plugins used about 10-15% CPU power. This will typically be the same for any PowerCore plug-in. Of course, the % of load on your CPU will depend on the type and speed of computer you have.