A large number of objects by itself doesn't automatically mean bad performance.
There are many things which can affect the performance - effects, behaviors, events running on every tick, collision checks etc. You need to find which of these factors are present in your game and try to optimize them.
If you have an effect applied to many objects, move it to a layer instead.
If lots of sprites have active behaviors, you should disable them if possible, when they are off-screen. Pathfinding, for example, may be very demanding, especially on a large map. Try not to use actions like "find path" or "refresh obstacle map" too often. There's also a much more performance friendly plugin for pathfinding you can try - EasyStarJS.
If the game makes lots of collision checks per tick (1000 or more), you can try reducing them, this will definitely help with the performance.