It doesn't matter what browser I use, Chrome, Firefox, Edge, or even on the android app. They all seem to start out fairly decent though not near as fast as they were a few weeks ago. Then fairly quickly become less and less responsive.
I wonder if it has to do with the new feature of the little easter egg rewards that pop up on around the city, shores and on the water. It seems to have started being slow just about that time.
I have Dell laptop core i7, 8GB RAM, Windows 10, Nvidia mobile GPU, SSD drive with 20GB page file.
I primarily use Chrome and fire fox. So when it started to get really slow I would have to reload the page and eventually completely restart the browser.
Not good when you have a lot of other stuff open on those browsers. So I started using Edge. Same story. But sometimes on a reload it just blips to a blank white page right upon load completion, like Adobe Flash crashed.
When I run it in FireFox I watch the memory for adobe flash and it just climbs and climbs. I saw it go over 1.6GB of ram (I do programming and computer support for work... that 1,600MB is accurate) before I killed it.
Edge seems to wrap the adobe flash so you can't see that it is flash running or it is using it's own flash implementation. But it to get to using a lot of ram.
I think a memory leak was introduced recently in the game. Just my guess with the limited symptoms I see.