I knew people would chime in on the stop feature. Like I said I'm not criticizing anyone for using it, I just don't need it. I always thread to a relief and have enough experience to do so at any reasonable speed. I do believe this would be a good system for someone just learning the single point threading process. Also when I'm threading I use a magnetic dial indicator on the ways and can stop the carriage easily within a thou or two. So I know right when I'm at the end of my thread. With lots of practice you'd be surprised how fluid the treading process can become. 250 rpm threading is really no trouble at all. Of course it depends on TPI as much as RPM. 32 tpi @250 rpm is quite a bit different than 4 tpi@250 RPM. If you are threading 13tpi @250 rpm the carriage is moving roughly 5/16 per second. Super easy to disengage the half nuts at that speed. Yes metric can be a little more of a challenge with a standard thread dial but it is possible. And besides you will never see a master machinist stopping the lathe during the threading process except when measuring with thread wires.

Sorry have to poke a little fun! Of course almost everything is CNC now anyway
As far as braking I honestly can't remember the exact amount of time mine is set to. I'd have to look to be sure but it's somewhere between .6 to 1 second. I agree that anything under 1 second is probably overkill and there aren't any advantages. I installed a foot brake on mine simply because I'm so used to having that option. It's literally muscle memory at this point for me and when it's not there it throws me off my game.
The braking resistor I happen to be using is 1000 watt which has nothing to do with braking performance, it just has to be large enough to dissipate the energy of the dynamic braking process. 1000 watt is insane overkill for my application, I just happen to get it for free. What matters is the "resistance" of the resistor. Mine happens to be 75 Ohms which is in the acceptable range of my VFD. (really should be 70) every VFD will have a specific resistance required. Can't just throw any resistor you want in there. Too high and it won't brake as effectively. Too low and you'll start smoking things like your IGBT's.
On my system I happen to be using a combination of dynamic braking and DC injection braking (DCIB). I work in the Automation Industry and we use this combination at the plant all day long. It really is the best of both worlds. I use DB to get 85-90% of the kinetic energy out of the system and then kick a little DCIB in to finish it off. It gives you really smooth braking performance. Especially when you have those heavy chucks you're trying to stop.
lathes with a mechanical brake do so with the brake mechanical setup on the motor itself which can stop the chuck quite quickly. I would argue this puts every bit as much of a strain on the system as any dynamic braking system. Lathes have had mechanical brakes on them forever so I'm not worried about any negative effects from this. I have never done the calculations but I wouldn't be surprised if the lathe operating at full load puts far more stresses on the gear train then braking at almost any level.
But like everything else in life what works best for you is what you should do. I'm only sharing my experiences of what works for me.
Hope I haven't offended anyone here.
Randy~