As you can see, most DSL customers are put on the Fast Mode first, since there’s a higher efficiency (more data, less error correction code in each packet).
Also, Fast Mode allows users to have quicker, smaller ping times.

Unless a higher gauge wire is used in the local loop, or a the power output of the modem is increased (for a greater number of frequencies) the max distance in the loop, to perserve sync is about 5.5 km. The sync would be roughly 230 kbps.

On the other hand, if the interleaved channel is used, due to poor wire or interference on the loop, a small amount of data is transmitted per packet, to make room for more error correction code. This means a lower efficiency exists, to get more reliability and less errors, loss of sync or dropped or lost packets. The ping times or delays would increase, as well.

Some carriers suggest that at 3.6 km distance, a subscriber could achieve 3 Mbps …however, real world conditions usually advise 2.7 km, from what I’ve read on this forum, for example.