I used to have a similar problem with lag due to the RAM being capped at 2GB for minecraft, and a game like that really needs more, however there is a way around this. Open up a notepad and copy the following text into it.
javaw -Xmx8192m -Xms4096m -jar "C:\Users\YourUsername\Desktop\minecraft.exe"
Just make sure that you have the exe for minecraft on your desktop and substitute YourUsername for the username on your computer. This above allocates 8GB of RAM for minecraft, its a good rule of thumb to not go above half of what you have. I have 16 so I went 8. If you have anything lower you can use the following numbers in place of the numbers in front of the Xmx.
so for example if you were allocating 3 GB of RAM you would put the following into notepad.
javaw -Xmx3072m -Xms3072m -jar "C:\Users\YourUsername\Desktop\minecraft.exe"
After you have the correct text inside of notepad all you have to do is save it as a BAT file. To do this just save as and then where it says Save as type, change it to All Files. Then in file name type in something like, MinecraftStartRAM.bat and save the file. Finally all you have to do is double click on the BAT file you just saved and it will run minecraft for you allocating more RAM than what it is manually capped at.
Hopefully this solves your problem, but if not there are other ways to increase RAM that I will post links to below. If what I link below fails I would just try to reinstall.