If your server has the right libraries the images are resized on upload so the public facing load time should not suffer.
You can add image sizes with add_image_size and retrieve those specialize sizes with functions like get_the_post_thumbnail or get_attachment_image_src, among others.
There are also a lot of post here about image sizes.
You may also be able to use small images in the slider but link to the large size.
You may be mistaken about something though. If what you mean is “upload images full size, use that full size image in the slider, and have it resize to fit when the page display”, that isn’t WordPress. That kind of resizing is done in the browser, by the browser, and page load time will suffer because there is no alternative but to send the whole image to the client. That takes times, and you add to that the CPU time need to manipulate the image on render.
As to whether any of that will work for you, I can’t say as you didn’t even mention the name of the slider, so it is pretty much impossible to say how it works or how to achieve what you want in the context of that slider.