I'm going through my 2nd allied race now. It's definitely way slower now, that's a fact. People who argue that have just no idea what they are talking about, or they belong to the crowd who "stopped using heirlooms while leveling because it feels better" before the leveling revamp. So now it might feel the same speed if they actually use them. As for the fun, it's a very subjective matter, that's the problem. Speed is objective, fun is subjective. They may have increased the fun for some, but reduced the speed for all.
Here's how I feel about different level ranges based on my Velf.
General thoughts:
1. You need to play the class you're leveling very well in order for it not to be a pain. For some people this won't be a problem, because they can pick whatever and just play it as if they mained it for years. Many others aren't like that, including myself. Unless you know the class inside out, the leveling will be notably slower. Some quests/bosses basically require you to max out your performance for the gear you own. And even then you may just barely do it
2. Many pieces of content are badly optimized. Quests, objective bosses and even dungeons. Here are some vivid examples that I remember from my velf
-Azjol Nerub - Hadronox. Those who made the mistake of doing this dungeon while leveling recently probably know already what I mean. The scaling of Hadronox and the mobs he has to eat is way off. It takes like 10 minutes of waiting for him to move up the slope and then he often evades or bugs out.
-The group quest in Grizzly Hills with worgen killing. I suppose this one isn't that valid of a criticism because technically it aims to be a group quest and does that very well. It's undoable solo due to the infinitely spawning adds.
-Mount Hyjal - druid boss in the southern part. Just flat out undoable without dying. The only way to do it now is to die halfway (usually more than once) and hope that you can corpse run while the mobs you summon are still fighting her.
-WoD bosses. Especially the boss you kill in the scenario required for building the garrison. This one is unskippable if you want to proceed and is now a challenge on the level of mage tower from 7.2. Especially if you're a class with a long cooldown on escape mechanics. You need to evade/live through about 3-5 attacks that hit you for 50% hp. Same goes for many bonus objective bosses in WoD, but I was mostly able to get them down solo, but again - mostly thanks to knowing my class well. With some I had to have help (like the one in the cave bonus objective in Gorgrond).
A couple other quests with ridiculously overpowered mobs, which sometimes causes them to evade and not work properly, which I don't remember.
3. WoD leveling on treasures is definitely faster now, the xp seems to be slightly higher.
4. Method-0 (hate the cringy name btw) sucks. And I'm not arguing with numbers here, people are saying it's significantly faster than questing, and it very well may be. The problem is that it's ridiculously boring and impossible to run for a longer time due to instance lockout. Maybe I'm still missing something here. I tried it in Tol Vir and it sucked.
1 - 60
This part is kind of okay. It is way slower, but the new way of experiencing the old zones is an interesting thing, until you level through all of the zones multiple times probably. So yes, people have a point that it's more "fun" at least in my opinion. The problem here is that the innovation will get old, and it will still be slow.
60-80
Hands down the worst part of the leveling now. Unless you are using some kind of exploit, which many people these days do. The problem is at some point they will probably 'fix' all those, and we will be fucked. This part of the leveling made me stop for about a month, because it was just that slow. Funnily enough, I was actually doing Northrend, which was stated to be faster than Outland by knowledgeable people.
74-81
Here I actually went and soloed Kara. Very fast and enjoyable. Wasn't possible for me below 74 which many people are suggesting. The mobs were simply too strong though that may be warrior-specific. After that though it;s been a breeze and I loved every moment of it.
81-88
Here I did Hyjal, simply cause I like Hyjal. Wasn't the fastest but I generally enjoy the zone so I was fine-ish with it, apart from some overtund quests. Also did the arena in Twilight Highlands for some quick experience.
88-90
Did the trash in ZG as suggested here by SirCowdog. It was okay, but I made a mistake of killing too many types of mobs. The best way would be to only kill the crocs probably, these give crazy amounts of XP. Next time this will be way more enjoyable cause I already know what to kill and what to avoid.
90-101
WoD leveling in general though is better imo. Save for the boss issues, if you can overcome these, it's faster and better than pre 7.3.5. Before, I could hardly reach 98 while doing all treasures in all zones. Now I easily get to 98 and return after getting the artifact and continue until 101 in Nagrand. Loved this part.
101 - 110
Logging in, doing an invasion, logging out. Leveling alts in legion has always been the best ever and remains so for now. I hope BfA will be as alt-friendly as Legion is.