So in summary, software 2019.12.1.1, older (MCU1) displays: turn off traffic display -> browser performance improves dramatically!
At the time v9 came out I read other reports on the same theme: displaying the Nav maps was crippling the web browser on MCU1 cars. It's suprising that turning off the traffic display makes such a big difference though. The other tips (pan the map so it doesn't update as the car moves, switch off online routing) didn't seem to have such a big effect.
Another workaround I've read is to bring up the sketchpad Easter Egg before opening the web browser - this completely hides the Nav so has the same effect as disabling the processor-hungry Nav features.
Trouble is, I like using the Nav and seeing traffic conditions. Also I find in v9 a lot of the time the browser isn't just slow - it completely hangs and is unresponsive. Hey ho, the browser has never been much good in MCU1 cars
I virtually never use the browser, but over the time I’ve owned my Model S the sat nav functionality has gone from awful to pretty damn good so better nav performance gets my vote over the browser. If I want to surf the internet I use my phone!