In summary, slow download times aggravate customers and greatly increase the likelihood that their visit ends in failure from your company's perspective. People hate horizontal scrolling. This Undeniable Truth is simple enough, and true for any computer interface. To explain, this web page is too long for you to see on one screen, so you have to scroll down to read it. That scrolling is vertical.
People have a very common, strongly negative reaction to horizontal scrolling on web pages.
Most web pages designs do not intentionally require horizontally scrolling, though some surprisingly do. For most web sites, horizontal scrolling occurs because a large table or image does not fit completely across low-resolution screens, such as those with 600 x 800 resolution. Stay away from horizontal scrolling when possible. At the least, do not go out of your way to include it in your design. People want a map, not a picture.