function updateSlidesVisibility() {

		// Select all slides and convert the NodeList result to
		// an array
		let horizontalSlides = getHorizontalSlides(),
			horizontalSlidesLength = horizontalSlides.length,
			distanceX,
			distanceY;

		if( horizontalSlidesLength && typeof indexh !== 'undefined' ) {

			const isOverview = overview.isActive();

			// The number of steps away from the present slide that will
			// be visible
			let viewDistance = isOverview ? 10 : config.viewDistance;

			// Shorten the view distance on devices that typically have
			// less resources
			if( Device.isMobile ) {
				viewDistance = isOverview ? 6 : config.mobileViewDistance;
			}

			// All slides need to be visible when exporting to PDF
			if( printView.isActive() ) {
				viewDistance = Number.MAX_VALUE;
			}

			for( let x = 0; x < horizontalSlidesLength; x++ ) {
				let horizontalSlide = horizontalSlides[x];

				let verticalSlides = Util.queryAll( horizontalSlide, 'section' ),
					verticalSlidesLength = verticalSlides.length;

				// Determine how far away this slide is from the present
				distanceX = Math.abs( ( indexh || 0 ) - x ) || 0;

				// If the presentation is looped, distance should measure
				// 1 between the first and last slides
				if( config.loop ) {
					distanceX = Math.abs( ( ( indexh || 0 ) - x ) % ( horizontalSlidesLength - viewDistance ) ) || 0;
				}

				// Show the horizontal slide if it's within the view distance
				if( distanceX < viewDistance ) {
					slideContent.load( horizontalSlide );
				}
				else {
					slideContent.unload( horizontalSlide );
				}

				if( verticalSlidesLength ) {

					let oy = isOverview ? 0 : getPreviousVerticalIndex( horizontalSlide );

					for( let y = 0; y < verticalSlidesLength; y++ ) {
						let verticalSlide = verticalSlides[y];

						distanceY = x === ( indexh || 0 ) ? Math.abs( ( indexv || 0 ) - y ) : Math.abs( y - oy );

						if( distanceX + distanceY < viewDistance ) {
							slideContent.load( verticalSlide );
						}
						else {
							slideContent.unload( verticalSlide );
						}
					}

				}
			}

			// Flag if there are ANY vertical slides, anywhere in the deck
			if( hasVerticalSlides() ) {
				dom.wrapper.classList.add( 'has-vertical-slides' );
			}
			else {
				dom.wrapper.classList.remove( 'has-vertical-slides' );
			}

			// Flag if there are ANY horizontal slides, anywhere in the deck
			if( hasHorizontalSlides() ) {
				dom.wrapper.classList.add( 'has-horizontal-slides' );
			}
			else {
				dom.wrapper.classList.remove( 'has-horizontal-slides' );
			}

		}

	}
Optimization method; hide all slides that are far away
from the present slide.