It's definitely not a game cache or device issue, nor is it an issue with background apps running. I have like 20 screenshots of the game freezing when I buy shipments, collected in the last several weeks. The game also has the hiccups on the PC app: less pronounced, but absolutely noticeable, so it's not even exclusive to mobile.
Please don't kick the can down the road by blaming end users, it's a problem with the app that is relatively new. I am 100% sure this didn't use to be an issue up until several weeks ago. I understand that the first, quite reasonable, instinct is to go "have you tried turning it off and on again" i.e. basic troubleshooting, but I've done that and the issue persists.
Edit: not to even mention that when a game that (at least in my experience) frequently literally freezes for upwards of 10 seconds at a time in battle, stealing game time off the battle clock, because it can't keep up with its own animations and the 374 effects playing off each other that need to be calculated in an instant, begins to choke when buying shipments, the reaction from users that it's a problem with the game is also reasonable.