In Coinomi you can manually choose how much you pay in fees. The transaction fees (default or custom) go to the miners of each coin and not Coinomi. There are 3 predefined settings (default/normal/low), but users are free to enter any amount they wish in "... / Settings / Transaction fees". You can get an idea of what fees to use with services like https://bitcoinfees.21.co/
If your transaction was sent with low fees, it can take many hours or even days for it to confirm. Once a transaction is broadcast, it is stored by thousands of nodes all around the world, so it can't be canceled. Eventually it will either confirm, or it will be rejected and the coins will return to your address like it was never sent.
Note on EXCHANGES from BTC:
Please consider increasing your fees when performing exchanges via our in-app exchange providers, Shapeshift and Changelly. The timeout limits of those exchanges might be reached if the BTC transaction was sent with low fees and does not confirm within 48 hours. In this case, please open a ticket with Shapeshift Support at https://shapeshift.zendesk.com/hc/en-us/requests/new or Changelly support at firstname.lastname@example.org and provide them with your transaction ID and a BTC refund address of your choice. (Most usually your current Coinomi BTC "Receive" address).