Just selected the 75% resource yield boost on a l6 silver tile. Duration for yield of 2000 troops hero and shaman went down from 7m54s to 6m34s not 75% on my calculation
Your hero and shaman (possibly) already gets so much resource yield bonus (skills, equipment) that the added 75% doesn't add much ... these reducers are better for troops that are on tile without hero/shaman ... I use them often when I am sending out a number of different marches on tile.