This is an issue we are not able to reproduce on our Macs in our studio. After searching through Google, many other games also have this same issue.
A common fix is to update OSX, though this does not appear to work for everyone.
Another common solution is disable your antivirus or allow/whitelist Chivalry in your antivirus settings.
Other solutions may be to manually update your video drivers to the latest if you are able to.