None of those were even fast dirt bikes either. Put any of today’s 450mx bikes and I’m sure they will be much quicker. Gosh I’d even love to see what a KTM 690 enduro would do.
The MX bike will reach top speed in about 60 yards while the street bike would build speed slower and end up passing the MX bike before the end of the 1/4.
Because it's filmed from a greater distance. Cars, motorcycles, even planes seem to go slower when they are farther away. If the cameraman would stand directly along the track, than you would see the real speed of the bikes.