What are the details behind Fists of Thunder's faster Spirit generation?

The tooltip is a bit misleading in this case. The only reason Fists of Thunder generates more spirit is because its animation is fast, which means you can use it repeatedly.

The thing about "on-hit" abilities in Diablo 3, is that there are very few static damage modifiers. Lifesteal is a %, and +damage is converted to % weapon damage by the skills as well. Hitting twice with Fists of Thunder will do 220% total damage, just like hitting twice with Deadly Reach (which also does 110% weapon damage). The difference is that you will perform two Fists of Thunder in less time.

As to how much faster Fists of Thunder is, that'll be hard until folks write tools to better read the game data, and it'll vary with weapon speed to boot. A slow weapon will mean slower Fists of Thunder (but still faster relative to other spirit generators with the same weapon).

Fists of thunder gets away with having a faster attack rate for the same '110%' because it has deficiencies in the AoE department. For fighting one monster at a time, it is probably the best of the first 3 primaries. On the other hand, you wont fight just one at a time very often.

Similarly, your on-hit effects will trigger more often in the case of single targets only.

It will probably beat 'way of the hundred fists' in spirit gain, but not in single target dps. As for How much it increases spirit gain over the other primaries, I would guess "a small, yet noticeable amount"