Koichi dreamt last night that he showed off his talent of throwing with a 150-kilometer-per-hour fastball.
It's pleasant but also fast. When getting on a train, it's nice to find a train that's both of these things. What makes it pleasant? That would be the high speed. What makes it fast? Also the high speed. Really, if you're on a train you'd rather get to your destination sooner than later, so a high speed 快速 train is the way to go.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.