There is a lot of work involved in monitoring HDV. With normal DV, whether it is standard definition or high definition, each frame is discreet so when monitoring, frames can be dropped, and apart from a lower frame rate, there are no other issues.
HDV however is very different. HDV frames are compressed so that there is an I-frame, which is a self contained frame just like a DV frame. However after every I-frame there are fourteen frames that are difference frames. You start with an I-frame, and then in order, each difference frame is applied, and importantly, the difference is cumulative. What this means is that difference frames cannot be dropped because if you drop one, then all frames will be corrupt until the next I-frame.
So what can you do to optimize Monitor Magic and Capture Magic HD if you are having performance problems? There are two option in the Preferences Panel on the Devices tab that you can use. The first one is sampling. This means that sampling for preview, waveforms, and all the other scopes can be done on a full frame, a half sized frame or even a quarter sized frame. This will help a lot. This even works for DV, not just HDV.
If you recall the explanation for how HDV works, then another option is to ignore the difference frames. This will lower the frame rate, but if you are trying to control lighting in a scene, then this is okay. To do this, select the Decode I-Frames only option.