Scrubbing through cached comp very sluggish

I have a comp that has been RAM Previewed and consequently, the entire status bar above the timeline is green.

However if I take my playhead and drag through the timeline, the screen updates very very slowly (perhaps 1~2fps).  But I can RAM Preview at full speed.

Any ideas? I thought once it's cached, and you aren't changing any parameters, it should scrub through because it's all loaded in RAM, yes? I swear I've done this a billion times in the past.

48GB Mac with 10GB free for other stuff. 200GB HDD Global Cache (although nothing in the status bar is blue. It's all green)