The sentence is “2x to 100x speed-ups, typically 4x”.
The x here is a way to write down the multiplication symbol ×. It means “times”, as in “speeds-up of 2 to 100 times, typically 4 times”.
The use of x to mark the symbol is a remains of the times typewriter were widely used, when the closest approximation to the handwritten multiplication operator was the lowercase letter x. Nowadays, all devices can cope with the multiplication sign itself (×), which is part of many post-ASCII character sets, and is of course included in Unicode as U+00D7 MULTIPLICATION SIGN.
It is a "multiplier". 2x4=8 (two times four equals eight).