If the renderer is very slow, smooth scrolling significantly increases the apparent interaction lag with the page.

I opened about a dozen tabs by middle-clicking on Google News, which resulted in them all being in the same process.  One badly behaved one caused very high CPU and memory use in the process continually.  This isn't ideal, but it's not a terribly rare scenario in my experience.

Trying to scroll any tab via e.g. PgDn resulted in a slow-but-reasonably-responsive (lag ~ 1 sec) initial scroll of a few pixels down, followed by a long pause (~ 5 sec?), followed by a jump to the final position.  I assume we tried to smoothly scroll, the initial movement triggered a lot of work, and by the time the next timer fired we were long past the desired scroll end time.

My ideas for fixing this are all variants on "keep track of responsiveness, and disable smooth scrolling while we're not very responsive".  This would likely involve timing the current message loop lag or latency in servicing previous scroll timer firings.