Though you can work around this limitation by creating your own proxies. Use handbrake or something similar to create lower-res proxies, put them in a subfolder in your project directory and use the media browser right-menu "swap video files" menu to switch between the two as needed.
This has the advantage (IMO) that VP will use your proxies no matter the resolution chosen in the preview window.
Just don't forget to switch to your full-res versions before rendering.