I've got an idea as to how you could fix your problem: as a temporary measure you could cap the CPU usage of 'whoopsie' by using a program called CPU Limit.
You can download it below:
This should ensure at least while watching movies that your film remains lag-free and once you've capped it, it shouldn't bother you until you next shutdown.I've found a well explained tutorial here if you need more information on how to install it and use it.
However as this isn't a permanent cap you'd need to get your Ubuntu to run it at startup each time if you want it to be capped on a more permanent basis. I'm not an expert on what commands you'd need to run but I think it should be possible to get it to run and limit the process at startup (see this post if you don't know how to set apps/processes to run at startup).
I'd suggest you try out CPU Limit and then if you're satisfied as to how well it copes with your problem and would like it to run at start-up you can use the instructions posted here by hhlp and by adding the following line:
cpulimit -e whoopsie -l 3 &
To the /etc/rc.local file it will run properly on every startup with Whoopsie capped at 3% cpu usage.
Ideally if it's a bug as commented it might be solved sometime soon, but until then this is the way I'd go about to stop the problem being an issue.