It's not an impression, rendering is definitely different, though not improved for all kinds of pages.

On the webmail I use at work for example (Horde/IMP variant), the somewhat HTML-heavy Inbox page used to load slowly but progressively, with visible activity on screen and the progress bar.

Now it "loads" an almost empty page immediately, and stays there for a long time (probably same as before), but with nothing visible happening whatsoever. The first couple of times I thought it was hanged and reloaded. But if I just wait, after a while the entire page just appears all at once.