I don't think FT8 is a good contest mode. A good CW op can do 4 QSOs per minute during a contest; FT8 is about 90 seconds per QSO. Even RTTY is faster than FT8.
Yes, the other JT modes are very slow, something like 5 minutes per QSO. So FT8 is much faster than they are, and that's the comparison that's made. It's actually even slower than PSK31, but it may seem faster because the exchanges are rigorously defined and don't waste characters. On PSK31, we're "trapped" by inefficiently defined exchanges, everything from multiple repetitions of callsigns (even after calls are acknowledged on both sides) to extraneous non-information that takes time to repeat. (For examples, dig through the reflector archives to the discussions on efficient contest protocols. I won't repeat them here because it always becomes a "don't tell me how to format my transmissions" argument, and no one wins. I wonder how many of those folks are using JT modes where they CAN'T format their transmissions?)
Where FT8 will benefit is working the weak stations, caused by either propagation conditions or just some stations' physical limitations (like compromise antennas). There will always be someone somewhere who needs "weak signal" capabilities, and FT8 (and other JT modes) fit that bill.