Improve performance with large result sets

[acheronfail]

At the present moment we go through the entire list of Items and render them all, even though comparatively few are actually rendered.

We can improve performance of rendering by simply creating empty list items for those items that won't be visible on the screen.

[acheronfail]

While this did improve performance a little, after inspecting the project with https://github.com/flamegraph-rs/flamegraph the biggest performance hit is the Printable trait. So we'll look there first.