Try completely uninstalling the app and reinstalling it from scratch first - not just the update, but actually uninstall it and redownload it from the Play Store. That often fixes weird bugs that get stuck during updates. Before you do that, make sure to link your account (Google or Niantic ID) so you don't lose your progress.
If that doesn't help, check if you have other apps running in the background that are eating up memory or CPU. For me it turned out that an old weather app was constantly syncing and that's why Pokémon Go was lagging. Also check your phone storage - if you have less than 2-3 GB free, the app can act weird even on newer phones. You don't really need a RAM cleaner though, that's pretty much just placebo.
Otherwise I'd reduce the graphics settings in the app itself. The A55 isn't weak or anything, but Pokémon Go is a badly optimized app. Less effects, lower resolution - try that out before you think your phone is the problem. And yeah, other people report stuff like this regularly after updates, so you're not alone with it.