Have you tried playing on WebGL version? Recently I have released an update that minimize occurrence of that bug, it still happen but much less frequently (it will be fixed completely in next update).
Also WebGL version has been optimized alot in last couple of weeks, for me fps nearly doubled on maps without vehicles and trees (ex. SecretLab, Bunker, Area56).
Unfortunately I will no longer update a WebPlayer version and it will be soon replaced with WebGL (probably next month).
Make sure that your browser support WebAssembly for best performance (Currently only Chrome and Firefox support it)
Check this link to see WASM support: http://webassembly.org/demo/
NOTE: WebGL version is connected to WebPlayer so it doesn't matter which version you play
When you open browser that does not support WebPlayer you will see alternative option at the bottom: