The phrase "initial rush" refers to the first burst or surge of energy or excitement that someone experiences at the beginning of an activity or situation. Full definition
But I think the effort was worth it because it generated that initial rush of downloads. (stevescottsite.com)
But after the big initial rush, it slows down to a crawl. (toughnickel.com)
Before you despair, know this: researchers have found that initial rush of feeling madly in love can only last up to two years at most. (poweroftwomarriage.com)