[Masters] Okessa's Doomsday

I'm sorry for the bad experience you've made with the deck. However I can assure you that I climbed all the way from Silver to masters with it. The playstyle might differ from most other decks currently in the meta, which is why it may be hard to pull off. I admit that this is not a deck intendet for easyly climbing the ladder, but more a fun build around Okessa ans Passionate Artisan that also happened to be good enough to get me to masters.
However, should you decide to give it another try, this is how I face each of the difficulties you mentioned:

Too slow: as is the nature of combo/control decks, this deck don't fair well against fast-faced aggro decks. The best option against aggro in this deck is using Dinosaur Nest and Passionate Artisan to hold off enemy units untill you can deal with them using Manacles or Avigraft. But I usually let enemy units hit me, as sacrificing about 15 life to play and activate Okessa on turn 5 is usually worth it.

Prone to color screw: I had some difficulties with this at first, especially hitting triple fire for the big finishers, but with the current power base I rarely run into any problems, since Okessa's audience can turn any unplayable card into the sigil needed, while Calibrate and Steyer's Eyes both help to search for required power cards. If you still run into power problems, I suggest adding few copies of Desert Alchemist and to exchange sandstorm scarf in the market for a three-color power card to really secure against color screw.

Winning condition doesn't finish: It is true that simply playing either of the big finishers doesn't end the game instantly,but both of them put you so far ahead, that it is practically impossible for your opponent to catch up. When cheating doomsday assembly into play through okessa, or even playing a copy made cheaper by Artisan, you can usually spend at least 5 power on ticking grenadins, instantly putting at least 30 unavoidable damage on the board. With Shavka's Embrace, you can gain immense advantage by playing either Tayana to re-draw the already cheaper relics, or Okessa to gain enough treasure troves to draw into a winning hand.

My general advice for anyone playing this deck is that you need to find the best way to get to your big relics quickly, according to your opening hand:
Steyer's Eyes + Merchant > Discard something big and play it with Unlock the Vault
Artisan + Oasis > Make relics cheaper and ramp to play them straight from your hand on turn 4 or 5
Manacles / Avigraft > Hold of your opponent while playing power cards each turn to stall long enough to get into your big relics
Okessa + Any big relic > Keep your units alive no matter what to cheat out your relics on turn 5
Okessa + Merchant > Pull Pit of Lenekta from the market to cheat out on turn 5 and secure your win that way
Tayana + Audience/Oasis > Reuse the Relics' sacrifice abilities to gain gard advantage, ramp and hold off your opponent untill you can use your big finishers

I am once again sorry if my deck didn't meet your expectations, if you've read this far and have any suggestions/improvements to help me make it more consistant, be sure to let me know, thank you :)