Same here. Trying out Logic X on a few different setups:
4-core Mac Pro with one 27" Cinema Display monitor: no video lag (by lag I mean, when you press play it takes 1-2 seconds to actually start playing)
Mac Pro (3.33 ghz 6 core) with two 30" monitors, 5770 card: some lag, not consistently, like every 3rd or fourth time.
Mac Pro (3.33 ghz 6 core) with two 30" monitors and 42" TV (HDMI), 5770 card: consistently lags. The strange thing is that we unhooked the TV to test it, but the lag did not go away.
OS is 10.8.4, Logic X 10.0.1 on all three. All three have solid state boot drives, session running from boot drive. I hope they fix this soon. I think it's to do with Logic using the newer Quicktime engine.
Does anybody think a more powerful video card, such as the EVGA GeForce GTX 680, might fix this issue?