I have my Slingbox connected to my router with an Ethernet cable. And when I watch the Slingbox with either the standalone Slingplayer application (on a PC) or via web browser, the playing is equally fast -- nearly 8,000 kbps -- with no jitter. So long as the computer on which you wish to view your Slingbox is on the same local area network as the Slingbox, it shouldn't matter which viewer type you use. I believe the browser-based system will figure out that your player is on the same local area network as your Slingbox, so the signal will be transmitted directly from your SB through its Ethernet connection to the router and then directly to your computer, staying within the local network.
What I don't understand from your original post is how you are accessing the internet -- and how your Slingbox is able to communicate with the SlingMedia directory server on the internet -- if you don't have a cable modem or a DSL modem. In other words, how is your ISP delivering Internet traffic to your router?