Jong and I tested adding this layer to the Explore Layers accordion and it is very fast and responsive. (see attached screenshot)

We can quickly add this layer to GLTG: app.config.layers

wms: http://isse.cr.usgs.gov/arcgis/services/LandCover/USGS_EROS_LandCover_NLCD/MapServer/WMSServer
id: 31
opacity: 0.3

Jong has asked that we show this legend somehow in the UI. Possibly below the slider, or in an Info modal popup. The response comes back as a PNG file. This will take more time to get this looking good.

http://isse.cr.usgs.gov/arcgis/services/LandCover/USGS_EROS_LandCover_NLCD/MapServer/WMSServer?request=GetLegendGraphic&service=WMS&format=image/png&version=1.0.0&layer=31