If you go into wireframe mode, does that alleviate the lag? By heavy, it depends on the poly count, and how much is needed to be calculated by the computer (dynamics etc), it can be much more than a video file, depending. By the looks of it, you've got pretty nice hardware.
Or.... do you mean that it acts all blocky because you are only scrubbing a few frames, and each frame is significantly different from the last, so not smooth. If so, then there is a simple fix. In the Prefs, go to the Time Slider tab, then uncheck Snapping. You will now have a butter smooth animation curve, because it's interpolating sub-frames too. Be careful though, that you don't key on non-integer frames, cause that can get messy.