I think ultimately this thing flopping is not one single thing but a cacophony of factors all coming together:
1) First and foremost: Ezra Miller. And no, i'm not talking about their controversies. The general public arent aware of that stuff...they arent aware of Ezra Miller in general and thats problem #1. Ezra is not a draw as a leading actor. He's no Tom Holland. He played a supporting role in a maligned movie (Justice League 2017) and a supporting role in another lukewarm series (Fantastic Beasts). Ezra is not a household name so your average joe has no vested in interest in seeing this person play The Flash.
2) The CGI. We're all mostly in agreement that this movie's CGI is inexcusable. I think the people of the GA who DID see this movie were turned off by it and are telling their friends and family (this movie looks like s*** dont go see it). Thus killing wom.
3) Crowded summer: May and June were CROWDED. And Thus this movie is not the only one thats doing badly. Elemental bombed hard as well and other movies are struggling. The Little Mermaid, a movie that by all accounts should have broken the box office wide open is also flopping. There's too many big releases in a short time span and audiences are getting burnt out on these things already.
4) The Flash just doesnt have enough cache (yet) with general audiences. This is the toughest truth to face but i think a relevant one. The Flash is not Batman, or Superman or Wonder Woman or Spider-Man. Yes, there was a fairly "popular" CW series that ran for 10 years but one that averaged, what a couple million viewers at its peak? The Flash to mass audiences is still an untested character and thus really had no reason to care about this big multiverse event movie starring this character. The character probably needed to be meticulously built up so people cared about him.
Now, some of you are saying "Kguillou, the movie is just bad and thus its performing bad plain and simple" but we've seen so many "bad" movies or movies that got dunked on by critics (Super Mario) do gang busters at the box office regardless or at least have a huge frontloaded opening weekend. My analysis above is about why WB couldnt even get people in the door to give the movie a chance because thats exactly what this low box office indicates. People just didnt go see it to decide for themselves if it was bad or not.