Users are complaining that a mobile app is slow. As a product manager, what will you do?

I would first check the feedback from users to know if specific actions or areas are affected and if specific user segments are affected like - android / ios users for example. I will then check performance metrics like load time, user drop off points, sessions durations to be able to quantify this issue. Once I gain this clarity I would work with cross functional teams to see if they are able to replicate or isolate this issue. I will also try to understand if they have made any changes at a high level - on servers / architecturally / used any new libraries / 3rd party tools that could have potentially brought this change. I would also review if the interactions from the users end have increased  and our servers are not able to handle this load.

Next step of action will be to assess the impact of this issue and prioritize this issue accordingly. If this issue is going to affect any of my business metrics like - user retention / user experience then priority will be high and resources will be reallocated accordingly to tackle this issue. I would urge the team to investigate the issue and I will figure out an immediate and long term plan based on what the team has found.