Sometimes I wish that 3rd Edition D&D rounds were defined as being 10 seconds long, instead of 6 seconds. The advantages would include that facts that (a) it would match the round length of original D&D once again, along with other early TSR games; (b) it would be more comprehensible that advanced characters are able to make so many actions and attacks in a single round; and (c) it would make the breath-holding rule simply the character's exact Constitution in rounds, instead of some multiple thereof.
In fact, there's very little to prevent a DM from just dictating that rounds are 10 seconds long, and maintaining all movement speeds and ranges at exactly the existing numbers. Breath-holding would last a dramatic shorter time (as above), and spells with durations in minutes would last only a multiple of 6 rounds, not 10, which usually wouldn't make too much difference in a typical combat (or perhaps it could be judged that they maintain a multiple of 10 rounds in their duration). There might be a disadvantage in that where one can currently convert speed-per-round to long distance travel simply by MPH = Speed/10, the new conversion would be a slightly slower MPH = Speed/15 (but, we can retain the normal conversion by assuming relaxed long-distance travel is a bit quicker than the base fighting movement).
Even though speeds are theoretically reduced, they would still see an increase over original D&D (which specified speeds for humans at only 20 or 10 feet per round, depending on encumbrance, in a 10-second round). The main question would be whether, at its base level, the speeds are realistic. Note that the average running human (30x4 = 120 ft/round) can complete a 100 yard dash in 15 seconds under the current rule, or 25 seconds under the revised 10-second-round rule. To match a world-record sprint of under 10 seconds, it currently takes a barbarian or 3rd-level monk with the Run feat (or any 6th-level monk even without the feat); under the revised rule, it would require a 10th-level or higher monk with the Run feat (who currently could complete such a race in just 6 seconds). Perhaps it wouldn't be so terrible to restrict this fantastic running speed to higher-level monks.