The top U.S. telecommunications regulator dismissed Verizon Communications Inc.'s defense of its policy of slowing speeds for some customers during periods of heavy traffic, saying he remains concerned that the purportedly technical practice has commercial motives.

Verizon announced in July that it could begin reducing its 4G wireless Internet speeds for its heaviest users when cell sites get congested. But Federal Communications Commission Chairman Tom Wheeler sent a harshly worded letter to the carrier late last month taking...