The indoor hard courts have been slowing down over the years and it's hit a point where they play exceptionally slow.
Not many players are really fans of that but it's an effort to make the matches more interesting as indoor tennis became quite boring with the rise of powerful servers in the game. Pretty much any top player now has a great serve and matches were mostly settled in tiebreaks with very few longer rallies.
John Millman is one of the players that questioned that voicing his frustration with it on social media. He asked:
"Can the ATP European indoor hardcourts get any slower?"