High faces mesh to Unreal

I am working in blender 3d to make some meshes. One mesh is really complex - has a lot of faces. It will eat perfermance in a game? or unreal will adjust faces somehow? What should I do to optimized it without losing the shape of the mesh.

Thanks )
But do I need to retopolegy mesh before or after unwraping and apllying textures?

for retopology it doesnt matter you basically creating a copy of the mesh but with less faces. for baking however you will want to have unwrapped and textured since you will be baking that information to the low poly model. theres a bunch of tutorials on youtube on how to do it. in some cases theres actually tools that will do retopology for you though ive never used them and you will get better results doing it by hand (well if you know what your doing that is).

its not really a unreal issue per se. what you need to first look into is retopology and baking normals. basically the idea is that you can take your high poly model and reduce the number of faces then you bake the lighting information (how light reflects) from the high poly onto the lower poly. if done correctly this will make the low poly model look exactly like the high poly one. the explanation probably doesnt make a ton of sense now but look into what i mentioned and you will learn.

beyond that there are LODs that you can setup in engine so when the mesh is far from the player it reduces its poly count.