I don't know why people are saying that's fast. That's slow to average in my opinion.
Anyways, if you could inform us what job you are and how fast you can kill harps/blood harps... That'd be helpful for us to be helpful.
I was a dexless thunderbreaker, and I got all my 10 peridots within 5 hours. I stayed at blood harps map only. Got 2-3 peridots, then changed channels, an collected 2-3 peridots again, and repeat. It may or may not raise the chances of the peridot drop rate by changing channels, but I think it may.