speed acts each n segments
level
0 10
1 9
2 8
3 7
4 6
5 5
6 4
7 3
Such concepts have been tried as part of DND in earlier editions, and
I'm sort of fond of them.
However: They really don't work well in concert with the game-balance
and mechanics of 3.0 derived rules. (Can't speak to 4th ed, but I'd
guess they are a bad idea there too.)
There are several mechanical complexities you have to deal with:
1. Open ended level mechanics. Your system is either going to break
down or wrap once the speed factor contracts to 0 or less. Are you
going to grant some sort of iterative attack? How does this affect
spell durations? Etc.
2. How to you map things that don't always fit the character level
model in? I.e. How do monsters work in this system? And mechanical
traps and things?
3. How do you map in combatants that join the fight late? (Orc
reinforcements show up in round 4... what's their baseline for
entering the fight when everyone else's initiative has shifted?)
... and lots of other stuff.
Like I said, I like the idea that some characters/entities are just
plain faster (or slower) and might occasionally gain (or lose) a round
of action as a result... But it's not really a mechanic that fits
well with contemporary rule sets.
Alcore