This slow reaction time is somewhat reduced when I run The Sims 4 from the NVMe on PlayStation 5. That is, some of the “lag” is probably due to loading/streaming animations from the HDD/SSD. If you notice your game lagging, freeze time in game (so your Sim don’t waste hours) and wait a bit for the loading to finish — that works for me sometimes, but it’s purely anecdotal. (Perhaps the Plumbob should have a throbber to indicate there’s loading/data streaming in progress, so we can do the freeze time and save some in game time by not waiting on the clock?)
About the incessant cellphone checking, some actions (I think?) are queued after idle animations determined by the environment and/or traits. For example, if you tell a Sim to feed the baby, first they will finish their idle animations (check phone, smell food, fan themselves if it’s hot) and only then they’ll perform the command you gave. If you’re lucky, the baby will still be there.