Deryl is not only a prodigy, he was studying bio-engineering prior to his empowerment. His transformation into a Superhuman was orchestrated by Ella, someone with deep knowledge of monster lore, and had a monster that (judging by Rebis Deryl's monster report) was a first generation Spawn (or at least creation) of an apostle infect him, (possibly) placing him in the same category as Alice Alderman in being an apostle grandchild. The other two first generation spawns we know? Aglaecwif and Valravn the (potential in Valravn's case) S-ranks. And unlike Jake, Deryl had the appropriate mentality and knowledge to drive his power growth, already knowing about Superhumans and tapdancing on the line between confidence and arrogance. Post failed evolution, his corruption allowed him to use his powers beyond his human capabilities, what he couldn't make with Elixir Ella and S.I.N. provided. He basically did nothing but experiment with and use his powers in his monster state, all while preparing for his ascension into a Rebis. I think it's completely justified how fast Deryl improved himself.
Especially when he technically wasn't actually that strong. He was hard-carried by his intellect, resourcefulness and trickery, fighting for his damn life and exploiting every weakness he knew just to stand a chance against people who had flat out the most broken monster parents possible. His monster parent is explicitly noted by him after the Battle of Diamonds to not be suited for combat. He got folded at level 1 despite having Jake also brutalising the MC and an expendable proxy acting for him so he could take bigger risks, folded in a rematch despite exploiting knowing MC's weaknesses, folded in his chimera state despite having massive amounts of prep time and tailor making a clone just to fight the MC and folded in his incomplete Rebis state. The guy did pretty damn well considering how much the deck was stacked against him. If anything variant-Deryl was robbed, and he was the one getting clowned on by plot. If I'd worked my ass off day and night developing my powers and custom-building hommunculi for power and combat efficacy just for a first generation apostle spawn to destroy my custom made chimera and its backup when I had him dead to rights, an apostle amp their granddaughter when I also had her almost done for against my other chimera, and two Apostles and a damn arbiter turn up and assist my rival in beating my ass I'd be calling straight bullshit.
