A dynamic map service generates map images on the server each time a request comes in.
and the ArcGISTiledMapServiceLayer are already cached images on the server.
In my tests dynamicmapservicelayers are usually slower because as the name says the server has to create them for you on demand. Some map servers (in our case) don't provide tiledmapservicelayers so you have to use the slower dynamic variant.
You can check if your server provides this tiled maps by checking this attribute on the mapserver description page: Single Fused Map Cache: false
