As fast as you can get them going. Well not quite.
You got to add energy so that the electron acheives such high speeds.
If you add approximately 220 KeV you can get the electron to move 90% of the speed of light.
But, if you need to make it move at 99.9% (just 9.9%extra) you need a total of 11 million eV!!
One, way to think of it is that the electron increases it’s mass as it moves faster.
Closest scientists have got to is approximately 99.9999992% the speed of light. (not 100% yet)
WELL I DONT HAVE THE ANSWER TO THE SECOND YOU GOT THERE BECAUSE IT IS STILL UNKNOWN.
HOPE THIS HELPS.
