How can I speed up my website?
Lazy loading PRO
In the Slider settings → Optimize → Loading you can either choose to load your slider after everything else has been loaded to your website, with the Delayed loading option, or what really makes a difference is the Lazy loading, which makes your slider only load as many slides at once as you want.
Don't use too big images, most people watch your slider with full hd (1920*1080px) or smaller monitors, so let that be the high end, your image's size should be as big, as your slider's size in fullHD. You could also turn on the Optimize images feature at the Slider settings → Optimize tab, so Smart Slider 3 would resize your images automatically.
A small image can have a big size too, if its quality is too good. After a given quality people's eyes can't see the difference, so don't use images with too high quality. You can lower your image quality with an image editor program like Photoshop or GIMP.
Also use JPG images, not PNG or other high quality image formats.
Number of slides
If you are using a lot of slides, don't be suprised, that they are taking a long time to load. You shouldn't use more, than ~6 slides to have a good loading time. You can see here, how to check out what is loading for how long on your website, and if you are loading many slides, this is just how much time it takes to load that amount of data.
Device optimization PRO
If you want to optimize the size of your website for tablets and mobile phones too, use device specific images, where you should choose a smaller image for tablet and mobile devices.
Or an even better option would be that you turn off your slider for mobile devices with the Slider settings → Slider size → Show on devices option, maybe create a new slider with only a few slides, put that into the same position, and make it show up on phones. Because for example on phones people don't look at sliders, because the screen is so limited, but rather want to just see the content, you could choose to not to show it at all.
Use cache in your website, but test it out thoroughly, because not every plugin or extension works properly with them.
You can read more here about the Joomla caches.
Still too slow?
Joomla: If your page load for more, than ~30 seconds, then go to the Extensions → Plugin Manager, filter your system plugins, and unpublish them one-by-one. If something is wrong with a code, that can really slow down the website.
If your page isn't that slow, just slow, google website speed test, and check out, what takes so long. Or check it with your browser by pressing F12, click on the "Net" or "Network" tab (it's different for browsers), and refresh your website pressing Ctrl + F5 (hard refresh). You could also use online speed testers, like https://tools.pingdom.com/.
Usually the problem is with the time to first byte, which means the amount of time until the first byte is received from the server, and that can indicate the slow speed of your server, and you might want to consider moving to another.