Try changing the web mode to UJS-canvas (experimental but stable) or Flash (legacy) to make Warzone run faster in your browser.
As for on your phone, reduce the FPS. To do this:
- Open Warzone
- Tap the profile icon in the top right
- Scroll to “Frame rate”
- Choose a lower frame rate by tapping on the current FPB
Which phones do you run Warzone on?
Which browser do you use and what are your computer specs?
You can always send a bug report by going to Help -> Contact -> Report bug if the slowness continues.