|Interleaving||Interleaving is DSL forward error correction by spreading your packet bits in time and interleaving them with bits from other packets. This is a feature designed for more robust video streaming (ADSL was originally developed for "cable TV on copper loops"), in theory a noise spike would have lower impact on data loss...
Since TCP/IP has it's own error correcting interleaving is not as important. Fast Path (fastcells, fast switched) disables or greatly reduces the interleaving "spread" of the bits, decreasing latency. Lower ping time/latency improves overall performance, makes browsing more snappy, and is critical for online gaming.