This movie is not slow. Goes straight to the point from the offset and there is no hanging around. However the director missed a trick - if the movie followed the script of the comics then the ending and the whole take on the movie would have had the potential for a blockbuster finale.
The alien race shouldn't have arrived to take Jean's powers or to kill her rather they need Jean to help them save their race from another more powerful galactic being. So instead of making Jessica Chastains character the villains the movie should have developed where the alien race and x men fight but evrtually must unite to face a even bigger problem. This would have left the series needing a dark Phoenix part 2.
The movie could have also built the emotional connection between Jean and Cyclops and little further. There were many moments where these could have been given the opportunity to do so but it felt like they were cut short. It's a 1 hour 55min movie but had they strechted to 2.5 hrs then the plots and storyline could have been developed further.
By the way - a decent soundtrack by the great Hans Zimmer.
If give it a 6.5 out of 10. No way as bad as official reviews are making it put to be.