Freeing up space · Caches and clutter
Why is an app slower to open right after I clear caches?
Short answer: The app is rebuilding the cache it uses to load faster, so the first launch or build after clearing takes longer. Speed returns once it's rebuilt; nothing is lost.
Because the app is rebuilding its cache. A cache exists to save work — downloaded images, compiled code, thumbnails — so the first launch or build after clearing has to redo some of that work. Once it's rebuilt, speed returns to normal.
Nothing is broken and no data is lost. If an app is slow every time, not just the first time, the cache isn't the cause.
If the slowdown bothers you more than the space, move that app's data to your fast drive instead of clearing it — see Can I stop one app from filling my disk?