I looked over your Xavius parse and also looked you up on Armory. You have way more passive haste than I do and I can still pull over 300k DPS on Xavius. Just as a disclaimer, I do have the Chrono Shard and Strand of the Stars combo and Mangaza's Madness.
From what I could tell, you're not using Void Torrent and Power Infusion often enough and/or at the right times. When you go into Voidform, you should be torrenting within a few seconds. I always VTor right after a Voidbolt. I also PI after VTor. As my ability to stay in Voidform has gotten better, I've started popping PI a bit later. I used to pop it at about 10 stacks, but now I pop it at 20. The objective is to get 100% of your PI inside of your Voidform. Try to hit 40 stacks without StM using PI and VTor. VTor may not do a lot of damage directly, but it will make your dots do way more damage if you have the Mass Hysteria trait.
You also don't use your manafiend. On your Xavius fight, you used it once and he did 869k damage at the cost of ONE GCD. It's up every minute, which means you could have gotten 6 casts off. You missed out on over 4M damage. That alone doesn't seem like much, but it all adds up.
By the way, don't be that douche that drops out of the Dream state early because he blew StM at the start. Stay in it, do mechanics for awhile, then blow StM and melt face.
I also noticed you're not using combat potions. Use them when you're at high VF stacks in StM. Are you flasking, eating 375 haste food? What golden traits have you gotten on your weapon?