D&D 3E/3.5 [3.5] Speeding Up Combat

I believe the quote from WotC was something along the lines of "8 hours of gaming in a 6 hour session" or maybe 12 and 10. Anyway, while I see a lot of changes intended to facilitate that (spells are now almost always an attack roll or a saving throw - fewer dice rolled; buffs give flat bonuses - changes can easily be accounted for ahead of time; spell area of effects regularize - easier to remember, so less time looking things up; et cetera).

One that doesn't seem to be working out is creature sizes. On one level, I like having all sizes be square. On another level, I'm finding that Large creatures are seriously cramping the environments. We've only played a couple of sessions under 3.5 thus far, so maybe I just need to get used to the whole "squeezing" thing, but right now I'm considering making large creatures come in (tall) and (long) varieties (because this would be easier than rebuilding all my giant castles and villages / underdark settings / et cetera). The (tall) creatures would take a space of 5 ft. and have a standard reach of 10 ft., while (long) would take a space of 10 ft. and have a standard reach of 5 ft.; end result of both categories filling/threatening a 15 ft. square region.

Is anyone else finding that the new sizes and squeezing mechanics are failing to work towards the goal of speeding up play?

I'm allowing PCs to "squeeze" into a 5ft space. So far it doesn't impact things too much (it does make their lives very difficult (they are only level 3).

Yeah, I will be house-ruling the sizes. It's not practical in my game to try to make everything fit into 5' boxes, let alone make everything square.

I haven't had any problems with the new sizes. If your big creatures are "cramping their environments", my guess would be that your environments are too small.

You mentioned giant castles. A fire giant is about double the height of a human, so its castle should be at least twice as large in each dimension. A 10' wide hallway in a giant castle would be wide enough to walk down comfortably, but uncomfortably narrow for combat. Two giants could not (easily) fight side by side there, just like two humans in a 5' wide corridor.

A human with a sword can cover a 15' wide gap-- he stands in the middle and threatens out to both walls. A fire giant (Large) can cover a gap twice times as big, 30', because he stands in the middle 10' and can reach 10' out to each wall. A cloud giant (Huge) can cover a gap three times larger than a human, standing in the middle 15' and threatening out to both sides.

It seems quite straightforward to me.

It's not that it's not straightforward, it's that it's awkward to convert existing material. For the true giants, yeah most things are already scaled up, but even then there are places, particularly places designed with structural defense in mind, where the defending giants will no longer have the room to easily manuever that they did under 3.0. Sure they can squeeze past each other while jockeying for position, but why would they have built their castles that way? The answer is that they wouldn't have. If I continue using existing material without doing something, the cramped conditions are going to lead to the PCs shooting fish in a barrel.

For ogre dens, troll warrens, and ettin lairs, places that are generally already cramped because they're unworked caves and caverns, things get even more cramped.