Consider these scenarios -
1) You are against an offensive Heatran running max speed and HP Ice. If you are slower, it will OHKO you. But if you are faster, you can put up a sub before it uses HP Ice, follow up with Protect, and continue this SubProtect cycle to stall it. Alternatively, you can just OHKO Heatran with EQ.
2) A similar situation, just with Ice Punch Lucario. If you're slower, it OHKOes you. But if you're faster, you OHKO with EQ (and Gliscor doesn't give a shit to ESpeed).
Hope that helped.
Also, that set is called 'SubToxic' for a reason. I've used that set in the past, and the way it works is you basically switch in on CC from Choiced Terrakion or EQ from Choiced Garchomp, then Substitute as the opponent switches. Now, Toxic the switch-in as they break your sub. Also, if you're faster, you can simply stall them out with Sub+Protect (a Toxic'd mon faints in only six turns), and if you're slower, switch to an appropriate check/counter.
In short, Gliscor IS supposed to Toxic stall stuff.
While we're on that topic, I'd like to ask myself that why isn't the 252 HP/44 Def/220 Spe Bold is the main spread instead of the current one? If going to 285 speed, you only get the jump on Jolly Mamoswine, and since it carries Ice Shard, I'm not sure how can Gliscor stall it out. So, 280 speed should be enough.