I love Unity but its performance is slow and it always lags when I multi task. I have 512 MB of RAM. Please help me, I want to do my work smoothly with Unity.
Here are your best options
Upgrade your RAM!
The more RAM you have the more Ubuntu can cache and the smoother your system will feel. Also, the offical requirements state that you should have at least 1 GB of RAM.
Use the classic desktop
You can switch to the classic desktop very easily. Just go to the shut-down menu in the top–right corner, and click "System settings". There you can change your default desktop environment.
If you can't upgrade your RAM or use the classic desktop you can always tweak your system. If you open the System Monitor you can see what programs use a lot of RAM, try to use others or try not to open all the programs you need at the same time.
If you have classic desktop
If you have some heavy themes or other effects you can turn those down. just go to System → Preferences → Appearance (If you have a classic desktop.)
If you have unity
Just click on the power button in your right corner and select system settings in there you can select Appearance.