I have a linksys router at home, so can't offer any help with the actual web interface you get, but I have never used DHCP because although it's plug and play, when you do get issues, it can be tricky to resolve because you're relying on what the router is dishing out
I would have the router's LAN ip address set to something like 192.168.0.254
then you can start setting all your PCs to use static IP, starting at 192.168.0.1, 192.168.0.2, etc. The subnet mask on all PCs will be 255.255.255.0 and the gateway and DNS will both be the same: 192.168.0.254 (the router).
Like others said, I'd remove the second switch for the time whilst you diagnose the problem, and then add them back in later. Just start off with a single connection (your PC) into the router, set up static IP addressing and take it from there.
I've often had DNS problems (NTL grrr!) but never speed issues. When you say web pages take ages to load, does it take ages to get anything, but as soon as the page starts to load, it loads up really quickly? If so, then that's typical of a DNS problem: it takes ages to find the site, but once it's found it, you get it loading up at 10MB speed
P.S Sorry, re-reading your post, I'm a bit confused. I'm trying to understand your physical connection. Do you have an NTL / Telewest cable modem (coax cable in, ethernet cable out into the safecom router) or does your router replace / plug straight into the fat coax cable?