HD videos can pose some problems due to their file size unless you have a really high performance machine and a great graphics card.  What kind of graphics card do you have?  But most importantly: Are you storing your videos externally or on your computer's hard drive?  That right there will screw up everything with HD: you need to have a RAID external hard drive to store your videos on or at least a decent Seagate or Western Digital 1-2 TB external storage device.   When you save your VPX "project", save it TO a folder in the external RAID/ Western Digital or Seagate hard drive.  If you don't do this, it will eat up your computers ability to perform completely.  Magic Bullet quicklooks is not a heavy program like the full version: your computer should be able to handle it and VX3/4 handles it with immediate rendering and without crazy GPU usage.  However, even if your GPU usage is extremely high, say, in the 90-100% range, its fine.  HD will do that and it doesn't hurt your computer.  Also, make sure your computer settings are for optimal performace when editing.  And make sure you don't have any background programs running either or other tabs open when you're editing.  

Also, go to your PLAYBACK options menu in VPX and make sure your playback mode is set to 1/2 resolution, not full resolution.  That will help a lot.  You can even set it lower if you'd like.   Basically, you have to optimize your computers performance to allow VPX to operate properly as with any professional NLE software and keep large files (all HD) off of your computers hard drive (export it to the external device).  Hope this helps.  Peace.  -Gene