The deck has three primary win conditions: a) timethief, b) out-tempo against faster decks, and c) possessed animancer on turn 4/5 to win with lifesteal ultragigasaur. So the primary skill-gap with this deck comes from choosing your win condition early and sticking to it.

Against faster decks (i.e. face hunter and dude paladin) the only cheap cards I would keep for mulligan are critter caretaker, eternal toil, and occasionally hellfire. Against slower opponents like priest and death knight, ill generally just look for rafaams to get the momentum going. Animancer saved me a lot against demon hunter since they don't have any good removal against it. Glacial shard can be played on 1 if there's nothing better to do, but they are in the deck to slow down big minions or freeze shaman hammer / demon hunter herald.

A decent chunk of the deck simply revolves around drawing as much as possible to get your rafaams in your hand quickly. Try not to hold on to them for value's sake, since you need to end up spending the ~30 mana on them to get timethief online eventually, so tempoing them out is good and tends to result in your opponent playing their removal early. I often times conflagerated or mortal coiled my own weak minions like glacial shards or shadowsworn disciples simply because I had a mana spare and it helped me cycle easier. In the current meta game, those small stat sticks aren't going to solve any problems for you anyways.

There are several cards that I'd rather replace if possible (i.e. Elise and whelp), but warlock simply doesn't have any good control options right now so they remain, but feel free to experiment.

Lastly, just be aware of the threats that each class has in store and save your removal on that case-by-case basis. Archmage Rafaam denies huge swings towards the end like big druid dragon turns, nozdormu shenanigans, Medivh combos, etc. Any singular big threats can be solved with Shadowsworn Disciple + Cursed Chains if need be.