Retopology reduces the geometry of the mesh to a lower-poly state. You've done that, which is already a huge step. This will decrease render times.
As you've surely noticed you've lost some detail during the retopology process, and that is a shame - but luckily there is a way to get around that: Baking a normal map. This will allow you to add the additional, fine detail in the material which will be vastly more efficient to render compared to the high-poly version of the mesh. This does require your low-poly mesh to be UV unwrapped - this tells blender how an image will fit on the model.
As a first step you want to move the models into each other - so that they occupy the same space (overlap, essentially). After that go to the UV/Image Editor and create a new image (Select Image -> New in the top bar). Select an image resolution. Set the background color to black and generate it.
After that you want to make sure that you're in cycles, and under render settings find the "Bake" option. Click the "Selected to Active" checkbox. Set the Bake Type to Normal.
Select your high-poly mesh, after that select your low-poly mesh while holding Shift.
Finally we need to tell blender where to bake the image to. To do this go to the material node editor and add an image texture node. Once it's been added select your image in the drop down menu. Select the node as well, and then hit the bake button. Yes, this is a somewhat confusing workflow.
Once you've hit bake you should see an updated normal map. If you get some weird green spots in your normal map you want to go and increase the "Ray Distance" setting in the render options.
If you want to see the workflow rather than read it I'd recommend this youtube video right here
