I think the best gear for a plauge knight speedrun
United Kingdom

Goo garment (for less friction) Fleet flask (for speedboost)

New York, USA

Goo Garment is neat but doesn't actually allow you to go any faster really. Perhaps a few niche timesaves but none that would make it worthwhile to get for a speedrun since you have to watch the whole Troupple King dance to get any armors.

Fleet Flask also has a few minor applications for going fast but it's no faster than bursting. It does see some use in 100% and NG+ though!

United Kingdom

oh, it feels faster and i thought it reduced friction, idk