Ask HN: Trying to understand a high latency scenario

This is a personal issue, but I'm hoping you guys can share some more general principles: I recently moved to San Diego and it took a while to unpack the router, so we hooked a laptop directly to the cable modem, and, as promised, enjoyed a blazing fast 30 mbps connection. After a few days, I found the router, and plugged it in. The connection suddenly took forever. 20 seconds on average to load a page. Speedtest showed 20 mbps a latency of 1.1 seconds. When I ran mtr, the ping times were consistently between 0.9 and 1.6 seconds. I was using an overclocked WRT54G v 5 (250 MHz) with DDWRT firmware, so I changed to a factory WRT54G v8 (240 MHz), which did no better. I went back to a direct connection, and the problem resolved (latency by mtr: 50-90 ms). I bought an AirPort Extreme last week (1.2 GHz), and ping times are still super slow (about 1 second). Changing DNS servers (internal, Time Warner, Google, Level 3) did nothing. The modem is a Motorola surfboard 5101. I'm not asking for troubleshooting (although Time Warner's folks have proven useless), but I am curious: why would my network latency as measured by mtr skyrocket with a router inserted, regardless of which router it is?