This classic duel is 3 minutes in duration...
https://www.youtube.com/watch?v=X3gfFVmw0kA That's a lot of rounds no matter how long your rounds are.
I don't think movement in combat is bad, but it should be interesting. When the characters are fighting in an enormous 100'x100' room but can move 200' per round (average Sprint speed in RMU), movement isn't interesting. They can move to any point in the room every round. They can move from out-of-range of any thrown weapon into melee range in a single round. They can move from out of range of a short bow or long bow in just two rounds. They can move from out of range of typical offensive spells (100') into melee range with the caster in half a round. Spells and ranged weapons are much less potent unless you break the round up into phases so you can target people partway through their movement. But if you're going to have to complicate things with phases, you might as well just have shorter rounds.
With a 5 second round, you can sprint 100' per round (dash 125'), so at least you are taking some penalties but you can still cover an awful lot of ground. You can probably move to support any of your allies with one round of movement. You need to dash to get from beyond spell range into melee range of the caster in one round; that means a caster needs melee protection and can become vulnerable really quickly, at least to a quick, lightly-armored combatant. It will take you multiple rounds to close with an archer, so you'll want to use cover or concealment to close safely.
With a 3 second round, it would be 60' sprinting (75' dash). At that level you can probably move to support any of your allies, but if you are spread out it may take more time. You will need two rounds to go from out of spell range into melee with the caster, so you need cover to run down the enemy mage, and it will take you quite a few rounds to close from out of long bow range. Conversely, the mage may be able to defend himself, especially if he has Leaving or other movement type spells in addition to offense spells. Indoors, you will often be able to cross the room in one round.
With 2 second rounds, 40' sprint (50' dash), you often will need multiple rounds to move to support your allies, and closing with an enemy mage will take 3 rounds, making it dangerous (to say nothing of bows). You can still cross small rooms in one round.
With 1 second rounds, 20' sprint (25' dash), you will almost always need multiple rounds to move to support your allies, unless you fight in close formation (becoming fireball fodder). Closing with an enemy with spells, much less missile weapons, is extremely dangerous or even foolhardy. The mage is definitely capable of defending himself. You will almost never be fighting in a space that you can cross in one round.
These assume melee attacks every round and spells are cast every 1-2 rounds. Missile rate of fire is its own debate... Personally I think it best (for balance purposes) if the rate of missile fire is approximately the same as the rate of melee attacks, or at worst half of that. (Could be lower if there are skills and talents to speed it up, just as you can use skills like multiple attacks to get multiple melee attacks.) I would take extreme speed-shooting rates as extremes, requiring special skills and talents (e.g. quickdraw, adrenal speed) to achieve.
Missile weapon ranges are at least loosely rooted in real weapon performance, but the considerations about spellcasting can be mitigated by changing typical spell ranges (100' is the default, is it a good number?) and how many rounds spellcasting takes. Both of those are totally arbitrary, there's no realism aspect, only fun and balance to consider. With 5 second rounds, I might extend typical spells to 120-150', personally, I think a mage ought to be able to get at least one spell off against a charging melee combatant. With 3 second rounds, I like 100'. Judgment call, though, depending on how dangerous you want your mages.
(Side note, I would address the crossbow issue by redefining a Light Crossbow as a crossbow that can be reloaded without use of a crank. That way it will be slower than a bow, but not ridiculously so. A Heavy Crossbow will probably still be ridiculously slow and require a crank, but at least you can use your crossbow skill with something and have a decent rate of fire. The attack tables, rate of fire, and range in RMU imply something only slightly lighter than a Heavy Crossbow. I mean, the Light Crossbow hits about as hard as a Long Bow, that's not Light. For reference, I can, just barely, load a 105 lbs draw crossbow without a crank or even a foot strap. Poundage is not directly comparable to bows, it's certainly not on par with a long bow made for war, probably closer to a 50-60 lbs bow, which I think puts it between short bow and long bow in power.)