I am currently playing with the RPG System mod that allows one to level player speed. I have noticed that once the the effect of added belt speed triggers the all other speed boosts are ignored. Disabling the immunity equipment and reloading or updating speed boost values will apply other speed boosts again.
I am not sure that is known or a technical limitation.

Thanks for reporting the bug.
I've checked the RPGsystem mod and indeed it alters "character_running_speed_modifier" which I also use in my mod. So for now, I'd say these mods are incompatible and I'll put up the dependency shortly to prevent this from happening.
A proper fix and removal of the dependency will come later.

Should be fixed in the version 0.1.3.

Thank you very much! I took a break from the game yesterday when I saw that this mod was now "incompatible" with RPG System, so coming back, seeing that you implemented a workaround so they work nicely "again" was a really good thing to see.
I really appreciate your work, thanks for your time!

Thank you for kind words! <3