it's not really a contest, nor should it be. Muay Thai is lethal because it's the most lethal martial art. It also has one of the most rigerous training regimens so someone who doesn't want that kind of taxation on his body or that big of a risk of physical injury should do boxing. Muay Thai is faster than boxing but it's nowhere near as fast as other martial arts because it focuses more on power than speed. This is especially true since all the kicking is done with the knees and shins, not the feet. As for wether a boxer could beat a thai boxer, it all depends on the skill level. Muay thai uses chinese boxing combos and (in my courses at least) you are trained to be completely ambidestrous with a stance that can be described as "opening a table" with both fists held up straight in front of the face. This gives the advantage of unpredictability. Furthermore, there is more of an emphasis on footwork and evading attacks since many of the kicks and elbows are powerful to the point of rendering blocking pretty useless (That's why i'm seeing a chiropracter for dislocation in the neck, jaw, coccyx, and pelvis, along with some jawline nerve trauma).