A bit more info is needed. It's akin to saying "When I put my foot down when driving, sometimes the car speeds up quickly, others times it takes ages". Well, are you towing something? Is it icy and you're not getting traction? Did you put a high quality/octane fuel in one time and not the other? Did you put you foot all the way to the floor each time? And so on...
Always give as much info as you can otherwise helping is pretty tricky.
- Are your speedtests consistent when things are fast and slow to load?
- Is anyone / anything else causing heavy network utilisation at the time you see slowdowns?
- Is you machine running any background tasks such as AV scans or updates when things get slow?
- Do you have any other applications open when it slows down that might be using resources?
- Are the pages you are loading the same between faster and slower load times?
- Are you using the same machine when testing, albeit both are windows 10?
I could go on, but you get the point. We don't even know which browsers you are comparing.