KMS only means Kernel Mode Setting, it's a way for the kernel to handle display resolutions so it can set the proper resolution for a display during bootup and enables instant TTY switching.

DRM is the direct rendering manager, it's mainly an interface for applications like X11 to talk to the GPU directly (ie 3D Acceleration and Video Decoding).

If KMS is causing things to be slow, it might be because the video driver is not working and the software framebuffer is being used. At higher resolutions that'll be slow.

If Chrome doesn't have the option to enable DRDC then the driver used likely doesn't support DRM or the window manager is not passing it properly.