honestly. it is very very slow that is true, but it also includes many other factors to include. His ult ignores terrain, you can legit flank the lane with such a wide cc ranged ult that not even a flash can save you. Hell, his ultimate's width exceeds max range thresh lantern and kalista ult, so there is no escaping unless you are at the very edge of it and not being pushed into it by other enemies.
Plus, lategame when you are already in the mids of everything, it is way too difficult to pay proper attention to his ult. yeah, if it hits early the root will be 0.4 seconds but it still is a root after all, one that can force the enemy to retreat, creating a very wide gap in their defence. OR it can catch their backline offguard and create an opening. His ult is very slow, and that does get very annoying, but his ult requires not efficient, but accurate use. You have to time it perfectly, create the right circumstances, be very precise with the set up. His ultimate requires a crap ton of Skill and preparation to use efficiently and it will take me at least 30 games until i can use it effectively but i have seen some people carry the lategame with just that R button on maokai ALREADY, and hes been out for about 2 or 3 days now