This does not, however, generate a push notification to your phone as it did uncontrollably for much of Plus' history. Even after the setting was added to control this, it didn't work for a long time.
What this really came down to was the lack of prioritization: the follower model is closer to Twitter than Facebook, being very public oriented, so you're far more likely to receive notifications about complete strangers but the UI copied Facebook's notifications. This meant that most users were trained to ignore the notification bar on other Google properties for a long time as everyone they'd ever exchanged email with was pushed into the service. Anything more than casual testing would have made it obvious that they should have had a way to group notifications so e.g. replies wouldn't be flooded away and to prioritize "added you" less than "added you back".
> You might want to verify whether the behavior you're seeing is due to a misbehaving browser extension or user script.
Nope - vanilla Chrome on OS X (also Firefox, Safari and Opera). Similarly, even on iOS where you can't add user scripts at all, the touch scrolling is jerky and buggy, requiring a scroll/wait/scroll cycle to get it to scroll to the bottom of a comment thread.
The default for being added to a circle is still to notify you regardless of whether you know the other party, which seems to match the behavior of most other social networks (people like to know who's watching them).
Scrolling is smooth for me in Chrome on MacOS. The spacebar pages down properly, and shift+spacebar pages up. You might want to verify whether the behavior you're seeing is due to a misbehaving browser extension or user script.