Quite the drop.
Also, the GOTG analogies are not exactly on point. It all comes down to expectation and needs.
Warners NEEDED this to be well received. They needed a hit that wasn't completely front loaded and would not suffer a huge drop off in week 2 (presumably due to bad word of mouth). They needed it as some sort of indication that they were on the right track and that Justice League (which will make or break, not only the DCEU but the studio's entire 10 year plan and the careers of several of its executives' careers) would not suffer the same fate. Basically, they needed some indication that BvS was the anomaly rather than the norm. Yet here we are, having the exact same discussion that we had 6 months ago. Despite basing the marketing strategy entirely around the second most popular character at DC's disposal, history is repeating and SS is having the same type of nose dive BvS had. But because Warners acted so hastily with JL, it is too late to course correct. THAT is why this is a big deal. Not because the numbers are bad per se, but rather because they are bad when looked at in context of what these numbers reflect for the franchise (the DCEU) based on the totality of the circumstances.
Conversely, GOTG was a sci-fi movie about a talking tree and raccoon. There were no recognizable characters (ala Joker) and it still managed to have great legs. But even if it bombed, Marvel did not NEED it. They tried something different, took a risk, it paid off, but if it didn't, it would've been shrugged off. Warners does not have that luxury.
Plus, this movie seems to be on the exact same pattern as BvS. GOTG was successful because good word of mouth gave it legs. It was earning in the 8 figures for 7 weekends (which is unreal). BvS (which SS has been performing nearly identically to) earned 8 figures for all of 3 weekends.
Again, this is important because it is a reflection of how audiences are responding to these movies, which is a reflection of how audiences will receive Justice League. You can point to a CinemaScore all you want, but these websites are not scientific polls. They are tainted by fanboys who go and vote (both ways) over and over. The general audience isn't doing that. They are simply telling their friends and posting on their social media accounts "that movie was ****, don't see it" or "that movie was great, go see it!" The ones that receive the latter have GOTG-typle legs. The former wind up like BvS and that is how Suicide Squad is looking. And that is a bad sign for the DCEU/JL, which, again, is a movie Warners has bet their studio on.