3.5 is very slow on-table, and it's hellish on the DM if you wish to prep accurately.
The DM's problem: Making a valid NPC is a lot of work - too much work, and similarly, monster abilities are all spelled out to a degree that makes subtle interactions likely and complex. The core premise of 3.5 is that a monster can only do what its statblock says, so the statblock includes rules for out-of-encounter abilities. That means that advancing monsters is very hard, but simultaneously, it's common for players to know all the monster abilities (since it's been around for quite a while, and since they need to for things like wild-shape). On the one hand we have a lot of player knowledge including out-of-encounter planning which can make this less fantastic, and on the other hand, we have a system that makes it hard for the DM to deviate from this. I spend multiples of the playing time prepping, and that's OK since it's sorta fun, but it would be more rewarding if I could achieve more in that time.
The on-table problem: the "simulation" of the in-game world becomes more and more complex as levels rise. Players gain more and more actions per round, and each action becomes more and more complex in and of itself. A first level character doesn't really have a full-attack action, since he can probably attack only once. A 20th level character can have many many attacks, a swift action, a number of free actions, and will likely have a bunch of "toggles" such as Dodge, Aura's, etc etc etc. All this means that the time it takes to play out one round becomes immense. Almost all combats in D&D are decided in less than 30 seconds (5 rounds), but they take a huge amount of time to play out. That incidentally also skews perceptions, since it means you focus lots of attention on combat, since that what you're likely to spend the majority of your time on, even if you enjoy role-playing, simply because it's so slow.
There are many factors which make combat slow down to a screeching halt. The increasing number of attacks is one. The increasing number of actions is another (immediate actions and delays/ready actions compound this issue). Even simple things like magic missile slow down if you down: everyone has one d4 die, but not necessarily 5. If you're adding 15d6 for a flame-strike at 15th level, that'll take longer than adding 5d6 for a fireball at 5th. Metamagic empower, which is generally more cost-effective, just makes things worse. Then you're got difficult abilities like grapple. There's the complex wildshape. Even in the new version, it's at least complicated since there's just so much to choose from and stat changes trickle down to so many different things.
There's more, but the above are easily fixable.
Which should we answer first?
