Crank up the allocated Xmx -- I'm assuming you're doing this on your local machine? I've seen the same behavior trying to use 48 columns.
The grid.less file is grinding away trying to compile when the client library rebuilds.
I've found throwing at least 8gb at the java process helps, but if you start AEM from the quickstart.jar in a local dev environment it will likely bonk on GC overhead.
Side note to changing the columns -- make sure you also change the number of columns in the Policy.