There isn't yet any master speed modifier that is open to .lua as far as I know.
Weapons can have individual modifiers but they only take the equipped weapon into account and not the character's loadout as a whole.
To use those individual modifiers you will need to first enable enableplrspeedmods=1 in setup.ini.
Then, individual weapons have gunspec settings for
plrmovespeedmod
plremptyspeedmod
plrturnspeedmod
plrjumpspeedmod
plrreloadspeedmod
The value range for those is a multiplier where 1=100% of the start marker speed value, 0.5 = 50%, etc.
Not quite what you're looking for but may be of some use.
Gigabyte P67A-UD4-B3, Intel Core i7 2600K, 16GB Corsair DDR3, EVGA GTX 970 SC, Win7 Pro 64-bit SP1, Primary monitor @ 1920x1080, secondary monitor @ 1024x1280