GPS Lag

Hi all. Haven’t been here in ages and have a question that I hope someone can shed some light upon. 
I have had a 935 for a long time and used it a million times. The first one I bought crapped out after about a year and Garmin service in Hong Kong sold me a replacement for about $100 USD. It’s worked the charm ever since. 
Whike I wear it all the time and use it for runs, rides and swims, the battery is still going strong and I’ve only had one instance of it randomly locking up which a soft reset cleared. 
Recently, I’ve noticed that my run pace seems slower (ok, I’ve not been the same after my third bout of COVID and running in the morning heat, as well as getting older do no favors in this respect), but I decided to borrow my partner’s 245 for a comparison. The difference was significant while running with one watch on each wrist. 
After 20 minutes, the 935 was 4 seconds slower per km than the 245. I decided to compare splits with the Footpath app by drawing my normal running route. Even though I take the exact points the kilos click over with a grain of salt with the app, the kilo markers are much closer to the 245 than the 935. 
They are both set to GPS/GALILEO and I’m not running in an urban environment. Just around our house in a 5km loop. No highrise interference. 
So, I am wondering if this 5+ year old GPS chip is causing this difference. The 245 isn’t exactly fresh tech but is a few years younger than the 935. 
Any pearls of wisdom from anyone would be appreciated.