If all you want to do is measure speed and distance with no extra hassle, a wired speedometer is the way to go. Also their black and white screens are very visible in daylight, whereas phone screens are hard to see in daylight, especially direct sunlight. They're also the most accurate, gps tracking can sometimes be a little goofy.

If you want to track your route via gps or something like that though, most old smartphones that people are just throwing out are capable of doing it, even without having cell service. You connect to wifi at home to download the app and upload the route you travelled, and it will do gps tracking on your route. As far as I've been able to tell.

Smartphones have been capable of acting like a bike computer for...I think around a decade? It often isn't hard to find an old one for free, one that would do the job without even having to pay for service. (Note that I tried out one of my old phones on my deck right now and it works well for gps, I haven't tried it out moving around yet.)