Google’s once-untouchable online-advertising operation took a body blow, hurt by mounting competition and struggles within its increasingly high-profile YouTube unit.

Google parent Alphabet Inc. in the first quarter posted its slowest revenue growth since 2015. The poor results highlight the risks for one of Silicon Valley’s biggest names in effectively leaning on one massive, if lucrative, business.

For all its myriad arms and efforts to diversify, Google remains essentially an old-fashioned billboard operation with a high-tech gloss—and it now faces more rivals.

The company’s results are an outlier amid what has otherwise been a steady earnings season in the technology sector. Peers like Facebook Inc. and Twitter Inc. previously posted strong earnings, while Amazon.com Inc. last week reported record profit that will allow it to pour fresh cash into improving its Prime membership program.

Alphabet shares fell 7% Monday after hours, with the drop picking up during the earnings call as executives declined to answer direct questions about the flagging growth. Nearly an hour in, one analyst, Ross Sandler of Barclays, audibly sighed. “I guess I’ll beat a dead horse on the deceleration,” he said.