If at their house I would warm in a pan of boiling water, it literally takes a minute or two. If ‘on the go’ I take a flask of boiling water and a jug and stand the bottle in the jug for a couple of mins
I use the Tommee Tippee bottle warmer and it’s easy to bring around and plug in or use the travel warmer flask if we’re out. Heats bottles to perfect temp in 5 minutes
Holding it under warm running water or in a jug of warm water is the only way I’ve ever done it x