How quickly does website activity get logged in lead record?

1. How quickly does the "visited webpage" activity get logged once a known user clicks on our page ( not coming from an email click)

There's no one answer to this.

On most instances, under 95th percentile conditions, it'll be within 30 seconds, or definitely fast enough for the triggers you want.

Under extraordinary conditions it could be slower, like 5-10 minutes, and I don't believe there's a guaranteed SLA that you could point to if you have periods with those longer delays. Nevertheless they should be rare or there's something up with your instance.

2. Our blog has infinite scrolling -- will this affect visited webpage from getting logged?

There ​is ​one answer to this.  If you have one big page, you have to manually fire Munchkin hits as your new sub-pages get scrolled. Any infinite scroll plugin will have the ability to fire scroll events, you then have to wire them up to the Munchkin API ​Munchkin.munchkinFunction("visitWebPage​")​ method.