I added an effect to a 3 minute clip. In the lower left corner, the Frame Render Time says 194ms. The green line took about 3 minutes to spread across the top of the entire clip in the timeline. The time doesn't seem to change at the bottom. Will it really take 194minutes?
Error or problem
Import and export
As noted by @Andrew Yoole green line indicates cache files for preview. As soon as you modify anything, the green line recalculates the frame. Cached preview files will be removed only if you use the Edit > Purge commands or if you close After Effects. Green line uses RAM for preview but sometimes you will se a blue bar that indicates cached frames stored on your hard drive not your RAM.