Swap Furious Charge for Threatening Shout - Demoralize and Brawler for Ruthless. Farm a Hellfire neck with Berserker Rage as your 5th passive (the point of Demoralize is use it to herd huge packs of mobs together in large areas). Change your Battle Rage rune to Bloodshed. I assume you're already using Ignore Pain IIB over Sprint for GR's. Use the Pain Enhancer over Gogok of Swiftness and try to snapshot a huge IAS Whirlwind early in the rift that you can use all the way including on the Rift Boss.
Those are the obvious things. That being said, most of it is getting fixed as of next patch so investing a lot of time into farming the Hellfire neck or leveling your Pain Enhancer is basically a huge waste.
Rift bosses will always take a shitload of time for you. The only thing you can do to maximize efficiency is to kill the trash even faster, then hope for an easy, slow boss with adds and a huge hitbox. Some bosses will not be doable for you even at 53. Once you reach GR58 or higher the only killable boss is Stonesinger. Even if you could instantly spawn the other bosses as you got into the rift you'd not make the timer.