As said you want the first charge to be slow, as recommended by the manufacturer. This is known as a forming charge.
Failure to do this can result in diminished capacity.
After the first charge you can charge faster but I would NEVER charge a pack in 20 minutes. Your batteries will not last long doing that.
I don't recommend charging at faster than 1C or .6amps for your pack but personally I always slow charge my packs overnight.