// https://developer.valvesoftware.com/..._of_L4D2_Cvars //
// Smoker //
//sm_cvar tongue_health 300 // Default: 100
//sm_cvar tongue_victim_max_speed 1750 // Default: 175

// Spitter //
//sm_cvar z_spitter_health 300 // Default: 100
//sm_cvar z_spitter_speed 2100 // Default: 210

// Boomer //
//sm_cvar z_exploding_health 150 // Default: 50
//sm_cvar z_exploding_speed 1750 // Default: 175

// Hunter //
//sm_cvar z_hunter_heatlh 750 // Default: 250
//sm_cvar z_hunter_speed 3000 // Default: 300

// Jockey //
//sm_cvar z_jockey_health 700 // Default: 325
//sm_cvar z_jockey_speed 2500 // Default: 250

// Charger //
//sm_cvar z_charger_health 1380 // Default: 600
//sm_cvar z_charge_start_speed 2500 // Default: 250
//sm_cvar z_charge_max_speed 5000 // Default: 500

some of them use the "z_speed" cvar, check.

Also the speed sometimes in only "applied" on a new spawn.

Btw u posted in the wrong section, would be better having it on "Scripting" or "Gameplay Ideas etc" area

you are correct, they also change the speed for common zombies, for some reason valve made they using the same cvar. (in case of some SIs)

If you wanna individual speed settings for each SI then probably will need to write a plugin, or maybe find something in the forum.