By putting the DSL modem in bridge mode, you remove a layer of NAT'ing (Network Address Translation) in your network. Bridging the router makes it act like a "transparent" converter box, so it doesn't have to process the packets coming to it from either direction... it simply forwards them to the next device in the path (either from the ISP directly to your router, or from your router directly to the ISP).
For the most part, it doesn't buy you much performance, unless you run your modem at near-saturation the vast majority of the time... which is highly uncommon for home users. Most ISP's don't run modems near their max bandwidth (you have a profile that is loaded into the modem by the ISP, which defines which speed package you are on... so the modem throttles your speed). So it's even less likely that you could even reach the rate of data processing where double-NAt'ing would be the first thing to impact your xfer rate.
Double-NAT'ing will impact your ping times, but only by a few ms... and it you are talking to servers on the net, those numbers are in the 10's of ms, so 1-2 more is small potatoes.