I just checked its not something you can do from the wallet, they are options you have to add to your zcash.conf file.
Here’s what the help says about those options :-
-dbcache=<n> Set database cache size in megabytes (4 to 16384, default: 100)
-par=<n> Set the number of script verification threads (-8 to 16, 0 = auto, <0 = leave that many cores free, default: 0)
You’ll find a .zcash directory in your home folder, the config file is called ‘zcash.conf’
Hope that helps.