Welcome to the VMware communities.
The RAM amount which you have allocated to the virtual machine might be a lot. The VM can run slow due to many reasons with the RAM being just one.
Other main reasons could be due to lack of CPU resrouces. I see that you have a Dual core CPU and the VM might be running low on CPU resources.
Please let us know the given observations when the slow performance issue occurs.
1. When the VM is powered on and running, what is the CPU usage on the physical machine?
2. How much free RAM is available on the host when the VM is running ?
3. Does it occur with any other virtual machine as well which is running a different guest OS?
You can find the above info in the Ubuntu Administration > System Monitor.
I can recommend few KB articles which would help you tweak the performance.
Troubleshooting virtual machine performance issues: http://kb.vmware.com/kb/1008360
Troubleshooting hosted disk I/O performance problems: http://kb.vmware.com/kb/1008885
Please also try the following:
1. Disable Windows Advanced Graphic features
2. If you have allocated more than 1 CPU core to the VM then reduce it to one core. Overallocation of resources to the VM, leaves less resources for the physical machine eventually slows down the host and then VM due to that.