For me it's a lot slower. I actually have a vid of someone playing KQ1 on a PCJR. In the vid, it takes about 48 seconds to go from the Sierra Online/IBM splash screen to the 1st player screen where you're in front of the castle. When I run it through MESS, it took about 1 minute 58 seconds to do the same thing. Same with going from one screen to another. It takes more than twice as long as I noticed it taking in the vid.
With the Coco version of the same game (versions might be different) the time period mentioned by EnvisagedOne has music playing. I'm pretty sure interrupts are used and that is more likely the source of the slow-down than floppy disk I/O.