That's certainly notable. I'm running a 7th-level 3.5 scenario where rounds last in gametime around two minutes (tops) with five players around the table. You're saying I can reduce that to 20 seconds or less?
Of our 7 or 8 players, we had 2/3 of them that were pretty new to D&D, so it took them longer to pick out which of their dozens of spells to cast that round or which magic item to invoke, as well as which buffs they still had in effect and how that affected their character... heck, if the party had time to prepare, it would take them a while to decide which buffing spells to cast between the cleric, sorcerer and psion, (as well as potions/scrolls) and what types of bonuses each buff did and if they stacked with other bonuses or not. Between potions, scrolls, spells and powers, each PC and bad guy having 6 or 7 "buffs" on them was not unusual. (I think there are 9 or 10 different categories of buffs: dodge, luck, divine, enchancement, armor, shield, deflection, and a few others that I'm forgetting)
Then, at higher levels, spellcasters often spent a good portion of combat casting dispel magics to try & debuff the bad guys, while the bad guys were casting the same to debuff the PCs. And, with each dispel magic, it necessitated a recalculation of the plusses/minuses to each affected PC or bad guy. Not to mention that some buffs/debuffs would not affect Touch AC, while others did.
And then - how much recalcing would be needed if you were wthin an Anti-Magic Shell or hit with a beholder's anti-magic ray...and then, next round, you roll out of the anti-magic zone and have to redo your stats again. Not only do you lose the plusses of the buffing spells, but all your weapons/armor/rings/amulets/boots/etc are now merely masterwork instead of +2/+3/+4. Is the PC who just lost the power of their belt of giant strength now heavily encumbered? Not to mention, his boots of striding & springing are just regular boots now. (And, then you gain them back again when you leave the anti-magic zone and have to redo some of the calculations)
Also, at higher levels, every spell and every attack is a potential game changer, so you can't really prepare an action too far ahead of time: if the party cleric is going to cast Firestorm, but right before her turn, a PC is dropped by a crit from one of the bad guys... does she continue her casting, or does she move to help the fallen PC? Or, does she cast something else entirely? Or, maybe she has something quickened prepared and does that as a quick action, and then proceeds to help the fallen PC. Or, maybe she can use the Quickened spell to help the fallen PC and then cast something else. Or, if the PCs are grouped just right, a Mass Heal can really help out a lot of people, including the fallen PC. Or, maybe she can still cast her spell and rely on somebody else with a potion or power to help out her fallen comrade. Quick, look at the positions of the minis on the battlemat & see who is close enough...