I'm not far off dial up myself. vB3 is best at around 3 seconds. vB4 tends to take 10 seconds (the reason I didn't upgrade), xenforo is in the middle at around 5 seconds.
Is first download speed a consideration for xenforo? If we can stay at 5 seconds for me I'd be happy. Worried you will add more complication for something to do once it's launched. If you made 10 seconds I'd go with IPB.