The largest issue I'm seeing on the phone end is in the Mercs subtab. It's possible to get so many entries that it starts freezing the game for a few seconds. If there are no tasks active, that's a few seconds of delay after each hiring. If tasks are active, it's roughly four seconds after each second of task completion. Full up completions are even longer pauses.
Seems counterintuitive - it's text and variable displays, you should be able to hold a billion of them simultaneously - but it's happening regardless. Maybe something to do with how Godot handles nodes?
In any event, it seems to scale with the number of potential merc hires. Maybe it's worth capping the unhired merc list at 30 entries, subbing out the oldest as new ones become available?