I don't think it is the game engine that is the challenge in itself as I said earlier, at least not the game engines limitations. I looked at search results from google from the sentence "game engine or developers to blame for performance issues", to look at some perspectives, and a few articles stood out to me,
Especially this one from 2009 that looks very relevant in 2018, which would probably give some insight into the process NWI is finding themselves in (would probably be a good read for NWI as well if not familiar with the content already, as it deals with challenges NWI is facing, and mapping some useful ways of thinking about it):
PDF download also available,
THE ONLY ARTICLE I STRONGLY BELIEVE NWI COULD BENEFIT FROM READING IN MY POST HERE IF NOT FAMILIAR WITH ALL CONTENT @Jonny-Higgins , @Iyagovos :
https://www.researchgate.net/publication/220686446_What_went_wrong_A_survey_of_problems_in_game_development
When it comes to engine or developer at fault, we know at least Fortnite and PUBG is made with versions of Unreal 4, and performs not too bad, video about optimizing UE4 for Fortnite 55mins:
https://www.unrealengine.com/en-US/events/gdc2018/optimizing-unreal-engine-for-fortnite-battle-royale
An article about how devs deal with bugs, not too interesting for NWI I would guess, but cool for gamers like me:
https://www.eurogamer.net/articles/2017-07-19-a-devs-eye-view-of-bugs
Here is an article that deals with the question game developer or game engine to blame, though I would say this article may be a bit shallow:
https://www.digitaltrends.com/gaming/dont-blame-game-engines-for-bad-games-developers-say/
And lastly if you wish for HL3 which we know might never happen, this mod is really impressive if you have not played it already, at least I enjoyed it a lot (It is a small game in itself within the HL universe. I actually bought Half-Life 2 episode 1, just to play it ( a requirement) as Adam Foster peeked my interest some time ago, expect about 3 hours worth of playtime @banani2804 :
https://store.steampowered.com/app/235780/MINERVA/