2 stops [2.8 - 2.0 - 1.4] ... to be precise.
Ok, hair splitter....several means more than one...
2 Stops is more than 6 clicks on a dial. It bumps shutter speed to x4 at teh same iso, which is a huge benefit.
So lets take an exposure of f1.4 @ iso 6400 @ f1/25th. at f2.8, I would get a shutter speed of 1/6th of a second....pretty slow.
I shot a wedding by candle light in a church crypt a few years back...f2.8 just wasn't possible. but due to f1.4 and f1.2 glass, I could get shutter speeds of 1/50th and 1/100th second at ios 6400. No flashes were allowed and during the service, shutter speeds of less than 1/50th would have been riddeled with motion blur by the targets in the photos.