The A15 has limited storage, so if you are running low on drive space, your device will slow down, as there is no room for a swap file. You can try clearing the cache of certain apps (social media, maps, web browsers), deleting apps you no longer use, wipe the cache partition of the phone (instructions on how to do so are all over this app and Google search)
And I watched some YouTube videos when they play roblox on same device and they are no lag and in some games it's not 90fps since this phone supports 90hz
