Returning player with fair amounts of experience needing help

Hi guys
I'm a returning player to Path of Exile and have been playing Betrayal League quite a bit. Managed to get myself quite a bit of currency too. My big dilemma now is that I've decided to follow DemiGodKing's Flying Purple whatever it was called Lacerate 2H build. All is going well and I feel that I've geared my character up quite properly.
I too have no issues mapping up to T15. However on the Guardians and T15 bosses I noticed I have been having some issues and thought that I may boil it down to my gearing once again. The PoB from Demi shows his Whirlwind Varian is 900k DPS but under the same conditions mine barely reaches 380-400K.
I was hoping that some of you more experienced PoE players are able to point out why my build is lacking the 200-300k DPS.
I'm on mobile and out so I'm unable to to provide a link to my character but my PoB can be found under ohtimo and the character is SonicThatGoesSideways
I appreciate all the help to help me find out what's missing and why I can't hit that 1m DPS and again I'm sorry for the inconvenience to you guys if you have to look my account up.

EDIT

Demi's PoB https://pastebin.com/SbEZzzpy
Here's my PoB https://pastebin.com/qV27qNd5

To be honest no one should follow Demi build unless he is in standard and has tons of currency to spare

First of all he has legacy items (show is POB as green). It gives a lot more damage than normal league items atm. Min-max wise (facing high level bosses) every bit of damage matter

Secondly your item choice of starforge limits a lot of damage compare to Demi. Demi use an insane 700 dps rare axe therefore he can use item to add % chaos/elemental damage on top of his physical. Your taste of hate gives u NO damage as Starforge deals no elemental damage.

Finally Demi gears (jewelry and gears) are mostly tier 1 rolls (which are non-existent in leagues) while your are good items but are not really as good as his. Remember, for build needs a lot of scaling like cyclone/flicker strike/etc, EVERY SINGLE MIN-MAXING DPS goes a long way for end-game bosses.

I recommend try other build instead of Demi. His builds are not special and often get carried by his tier 1 gears rather than the actual build. You can see builds from Mathil kills Uber Elder with shit gear (like 2ex max worth)