LATEST ** got a new version for 1.9 - I think speed is comparable to before but let me know if I'm way out **
The mutator is "HardcoreFasterSprint". The hardcore sprint speed is raised a bit, but it's not as much as in normal mode (I think). Walking and sliding speed is not affected.
I have disabled the hardcore movement animations. Originally this was a mistake. When I enabled them, it appeared to stop the mutator working as intended. I give up.
I believe you need to add "-mutators=HardcoreFasterSprint" to a server command line to make the mutator run (or else add it to an existing list of mutators). You also need to update your mod IDs (usually in mods.txt) with the ID of this mod (119336). You also need to add a parameter of the form '-ModDownloadTravelTo=Oilfield?Scenario_Refinery_Checkpoint_Security', otherwise you will need to restart the level to get the mutator to start operating.
I'd like to refine this so you can either use a console command or edit an .ini file to set the speed multiplier. Unfortunately the current system for modding appears to require the speed to be hard-coded into a data file. This means that changing the speed on the fly (or at all) does not currently appear possible. If that changes, I will try and update this mod to be a bit more flexible.