Can't say from personal experience as I've never fed a pen of straight Herefords against others, although a couple different feeders I inspect at have told me they're leary of them unless they're the right type. They say too many straight Herefords took far too long to finish, kept growing and growing before they filled out, much like high percentage continentals. That was their complaint - pure economics in cost per head.
Although I'd be the first to point out that the right type of any animal - shorter stature, more girth and guts that frame and flash, will always finish quicker, regardless of breed or lineage. I brand inspected some yearling heifers at the University of Alberta Research Ranch in Kinsella this morning. They've sold their open heifers to a feeder, and most were big leggy purebred Charolais heifers. Real raw-boned type, more leg than body, terrible looking critters. The story behind them was that they came from one research farm to this one, and have had a very hard time adjusting - obviously.
The manager there said when they fatten their own calves, it doesn't matter what mix they are, the same phenotype always rises to the top. Their herd has had many different breeds mixed in over the years with various breeding projects conducted through the University. Everything from Hereford, Angus, Galloway, to Charolais, Simmental and Dairy Fleckvieh. The one things that's stayed a constant has been the management of their grass, always using rotational grazing and stockpiled forages. The mother herd now looks very similar in type, just a wide range of colors. Just another little test plot that proves it's not about breeds, it's about the right type.