A recent update to the original XCOM: UFO Defense might have improperly changed a configuration setting in the Dosbox settings. If you start the game and it moves way too fast to play, then change the CYCLES line in the DOSBOX_XCOMUD.CONF file to this:

cycles=auto limit 16000

and it will fix the over speeding. I sent a message to GOG.com to ask them to fix the installation files as well, so hopefully it'll be changed soon.