Three things:
I also - even with my brand new phone - sometimes get a fleeting 'no GPS' signal. Pokemon Go is really good for telling you. I never get it in Google Maps or Maps though. But that does suggest something might be momentarily interfering with it, as you suggest. Every new version of iOS is more of a memory hog. I have a 6s and it's fine with it. But it's slowing my husband's 4s to a crawl.
Every 3 minutes for an update on Waze or anything else is just not acceptable. Driving on the highway, fine. But in the city? I was ready to throw it out the window. It works like that when GPS isn't working and it's getting triangulated signals from cell phone towers. Useless. My phone worked like that when it had a bad antenna.
For diagnosis: In Maps, you will see a blue circle around your location. It pulses every time the antenna pings GPS. If it's not pulsing, it's not pinging.The size of the circle tells you how exact the location is. When my phone was broken, the circle didn't pulse and was very large - blocks and blocks - most of the time. Sometimes it would get a good signal and get small. Mostly it was huge. And my location would be wrong all the time. It would consistently showing me at a house half a mile from where I live or on a bike trail half a mile from where I live. That was within the location circle, but not where I was.
You've probably read about those people who have houses that always get shown as stolen iPhone locations? I bet they are on those cell phone axes where my phone is shown that are just wrong.
In Google Maps, you see the circle, but not the pulses. The size of the circle again reflects the accuracy of the location.
Third, people keep saying lots of us are complaining so that proves it's not mechanical. I don't think it does. I'm a statistician. There are how many iPhones out there? How many have antenna problems because - like mine - they've been dropped? How many got dropped within a week or two of an upgrade? If you do a search you'll see with every upgrade people say their phone broke because of it. I'm going to suggest a lot of those are coincidences.
Plus - you're absolutely right - every time the iOS becomes more demanding on the phone, it brings out more problems, particularly if you don't have a lot of memory. Just my 3 cents.