If the VFX shots in recent movies look like the were done in a week, it's probably because they were.
The VFX companies place bids based on the number of shots ordered by the studio for a sequence. ( Ex : the Studio needs 3000 CGI shots for one sequence.)
The problem is that the amount of work per shot varies dramatically. The studios couldn't care less though ; to them, a shot is a shot.
Studios don't give a shit about quality CGI. All they care about is pleasing their shareholders and increase the output of superhero movies and reduce the time it takes to make them. They need speed over quality.
That's it.
Working relentlesss hours, doing overtime everyday and meeting unrealistic deadlines are the main factors that flush the VFX companies' performance down the drain.
The suits simply don't give a fuck.