Check out my thread here, about halfway down. Linux mint uses the Open JDK ( an open java application) by default, and has some bugs and other issues that need to be worked out. I found using Oracles' Java to help solve this for youtube and other sites wanting java. viewtopic.php?f=42&t=122822
While this is for the Opera browser, it can be implemented on any web browser, firefox included if you want by using that hack halfway down to purge yourself of Open JDK and install Oracles' java run time environment on your linux mint build. I found that workaround worked for me on getting rid of lags using the java flash.
I am using Vmware workstation 9.0 with linux mint also, using AMD 64 bit. Something about that Open JDK that does not run well under virtualization just makes it lag. Probably not optimized that way.
If you are getting lags on ubuntu under virtualization check your swap memory settings in your virtual box and UP the RAM to half of your availible RAM you got on your machine. Make sure it is set to 'allow memory swapping between the host OS and the guest OS.' That worked for me on defeating all of my lag issues when I got VMwares' hypervisor up and going. Updating your kernel base helped also, the new 3.7.6 kernels have kernels that help virtualization. You can update your kernel base at this link here I made in the forums for a guide. viewtopic.php?f=42&t=124819
Also make sure you got intel VT or AMD-v /RVI enabled in both your BIOS of your host OS and in your options for your hypervisor. That helps ALOT on lag issues. Your notebook specs though, I would rethink on using a hypervisor on it. The processor and the available RAM are just not up to specs. You should have at least a 4 core processor and 8 or more GB of RAM on your workstation or laptop to run a hypervisor efficiently. That 2 core processor you have , IDK if its got the specs to run a hypervisor on it.
You should look over a "bare metal" hypervisor for that laptop, like XEN to get rid of your lag issues. Harder to set up but you are not going through software bugs and lags like a software hypervisor like VMware or virtualbox.