relaxed toxapex allows slowbro to outspeed you and quagsire speed ties. most of the time ur just gonna toxic them both, recover off the damage dealt (in quagsires case its not gonna do much) safely and then just go back and gain health back with regen. its easier to do this since ur not gonna risk getting outsped twice and hit with supereffective stabs (in slowbro's case, if its running psychic, i think it has a legitimate shot at a 2hko) its not that important since both of them arent used as much (specially slowbro), but still its a nice advantage.
plus no point in losing speed when ur not running a physical move. if for whatever reason u decide to use liquidation or something like that then just run impish.