Users report the app gets slower the longer they leave it open. How would you track that down?

That pattern points to a leak or unbounded growth. Take heap snapshots in DevTools at two points after the same user flow, compare retained sizes and look at what is holding the detached DOM nodes. The usual causes are listeners and subscriptions that are never removed, timers that keep running after unmount, caches that never evict, and closures capturing large objects. Fix by tearing down everything a component created.

I would first confirm it is memory and not something else, so I would leave the performance monitor open and repeat the flow the users describe, watching the heap and the listener and node counts. If they climb and never come back down after a forced collection, it is a leak. Then I take a heap snapshot, run the flow ten times, take another, and compare by retained size. Detached DOM nodes are the giveaway, and the retainer path tells me what is holding them. In practice it is almost always something set up without a matching teardown: a resize or scroll listener on window, a socket subscription, an interval, or an observer that outlives the component. I found one where a chart library attached a listener per render and only removed one on unmount, so the count grew with every navigation. The fix pattern is boring but effective: anything created in an effect returns its cleanup, and any cache gets a size limit rather than growing forever.