Canon’s DPP is generally the faster tethered solution for Canon cameras. If Lightroom 4.2 is lagging after a few rapid shots while DPP keeps up, that points more to Lightroom’s tethered workflow overhead than to your camera.

A practical workaround is to tether with DPP for capture and preview, then have those files saved into a folder that Lightroom watches/imports automatically. That gives you DPP’s faster capture performance while still keeping a Lightroom-based workflow.

If you’re shooting several frames quickly, camera/computer buffering can also contribute, but based on your comparison, Lightroom itself is the main bottleneck here.

So the short answer is: there usually isn’t a Lightroom setting that makes it match DPP for Canon tethering speed. For faster, more reliable tethered shooting on a 50D, use DPP for capture and let Lightroom pick up the files from a watched folder.