Normally, it takes a long while before an egg that you carry around with you will hatch. However, you can increase the hatch speed if an active party member has either the Flame Body or Magma Armor ability. You can further improve hatch speed by hopping onto a Tauros and riding it around for a minute or so. This works best if you find a long, straight path, such as the lengthy stretches in Route 6, or at Heahea City on Akala Island. Running in circles also works, but requires more effort.