Slider loading issue

This is standard behavior with javascript that is deferred or loaded in the footer.

All the slides load in, and don’t get told to “be sliders” until the whole page has loaded.

You can hide them with CSS first:

#section-slider .single-slide-wrap {
    display: none;
}

And you can have the first slide be shown if you want:

#section-slider .single-slide-wrap:first-child {
    display: block;
}

Notice the same thing is happening with your .tab-pane‘s further down the page, so you could add those in as well:

#section-services .tab-pane,
#section-slider .single-slide-wrap {
    display: none;
}

#section-services .tab-pane:first-child,
#section-slider .single-slide-wrap:first-child {
    display: block;
}

Since the CSS is loaded in the header, the styles will be applied well before the javascript is loaded and kick in.