The VIC-II "steals" CPU cycles every 8th scan line in order to load sprite & color data, what C64 folks call "bad lines". It has a BA (Bus Available) pin that's fed into the address decoder PLA. When BA is low, the PLA tells the CPU that memory is only available for writing.

If you disable the VIC-II's output with a poke, it doesn't need any "bad lines" to fetch data. The CPU then gets 25 scan lines' worth of time to crunch data each screen refresh.