Slowing down helps you pick down targets better, specially the deeper ones, and it also lets you get around targets near trash and recognize them as good targets, being a DD coils (the coils for the Explorer) you have the advantage over monoloop coils, but slowing down is necessary, trust me, and why speed up? If you need to cover ground fast get the 15" WOT or one of the EXcellerator coils from 12.5" and above from Kellyco to cover ground quicker. If it's just you who wants to go fast go right ahead, leave some of the good targets behind. You need to overlap your swings, this means you need to go slow... All this is in the manual, didn't you read it?
As for the detector sounding different, no, it doesn't, the only thing that could change is the depth reading being a tiny bit innacurate, but just a bit in most cases.
www.sunraydetector.com (X-series coils and lots of great accessories for the EX.2)
www.kamakazi.com (WOT, Joey, etc. Coils)
www.kellycodetectors.com (EXcellerator coils)
Hope this helps, good luck!
Just one more thing, get the book "Mastering the MINELAB EXPLORER XS & S" by Andy Sabisch, it will aid you greatly. And if you have the money get the X-1 probe it's the best thing you can get as an accessory for your Explorer.