Spend some time playing around with PCem, and you'll find that the emulated machine runs at the same speed as the original hardware. In fact, PCem emulates ever

Spend some time playing around with PCem, and you'll find that the emulated machine runs at the same speed as the original hardware. In fact, PCem emulates every facet of the hardware: you're not just running a DOS environment, you're emulating an entire computer from end to end. You may not want to sit through an XT boot sequence every time you want to fire up an old game. Accurate emulators like PCem serve an important niche which is very different than quick-and-dirty environments like DOSBox. PCem and similar emulators aim to preserve thorough documentation of the hardware ("hardware preservation project" is probably a better term than "emulator" here), and they're also useful for running specialized software that doesn't work unless the emulation is 100% accurate. But if you just want to play a random old game, and it doesn't take advantage of specific hardware quirks, you'd be better off with DOSBox. Edit: Also, PCem needs BIOS ROMs to work, while DOSBox doesn't. This makes it easier for companies like GOG to ship DOSBox; if they shipped PCem instead, they'd have to either illegally include the ROMs (which will bring down the wrath of IBM's lawyers) or leave ROM acquisition as an exercise to the user, who won't be pleased at having to hunt down ROMs to play a game they bought on GOG.