Slow loading page

Hello,
My website is loading very slowly. I know my images are way too big now so I am going to reduce them (I have them on the 3000x2000 for the full page slider). What is the best size to have these at to not loose quality when viewed on large screens?
Also will the amount of photos I have on the slider contribute to site being slow to load? Is there a recommendation?
Wondering if anything else such as unpublished articles, modules make performance slow? Or as long as they are not published are they fine to keep in the backend?

Also is there a way of putting icon on the pages to indicate things are stil loading when someone looks at our site (eg a hour glass icon, or timer icon etc).

Thank you

Slider image in full width dont have to be full size like 3000px width.
We recommend a width of max. 1400px with 72dpi.

great thanks. what about the amont of photos in the slider, will that contribute to it loading slow or once i drop them to 1400 will that fix that problem? I have 35 photos at the moment... maybe too many?

WOW, 35 images are too much!
The frontpage need only max 3 images to get a good loading time.