How Can I Tell What’s Making My Android Phone Feel Slow?

My Android phone has started lagging when I switch between apps, but it doesn’t happen all the time. Storage is getting close to full, and I usually have several apps open, so I’m not sure whether the slowdown is from low storage, limited memory, or something running in the background.

Is there a reliable way to narrow down the cause before I start deleting apps or resetting the phone? I’d also be interested in which built-in settings are actually useful for checking this.