Eclipse Neon/Mars is very slow/hangs on a fresh 16.04 install and Upgrade

That works fine, you can also set:

--launcher.GTK_version 2

in the beginning of your eclipse.ini file.

This way if you have a .desktop shortuct it will also work automatically

I found the solution when I was on my main machine, I'm now typing this on my tablet, so please excuse the brevity. If you type export SWT_GTK3=0 before invoking eclipse from a terminal, you will load quickly. Googling for 'eclipse' and 'SWT_GTK3' will find the bug report that led me to the solution.

open terminal and type following command

 export SWT_GTK3=0 

add this to your ~/.profile file to make it persistent across logins (you'll need to logout and log back in to be able to start eclipse from shortcut)

then go to your eclipse folder and search for eclipse.ini file
Right click anywhere in the folder and choose "open in terminal" and type following command in terminal

sudo gedit eclipse.ini  

and add the following line before --launcher.appendVmargs

--launcher.GTK_version 2  

Search in the file for 256 and replace it with 1024