BACKUP_FILTER 'C:\Program Files (x86)\Pigz\pigz.exe --fast --processes 3 --blocksize 512'
RESTORE_FILTER 'C:\Program Files (x86)\Pigz\pigz.exe --decompress'
Fast compression is enough, there is not much difference between --fast and --best. (--best needs a little less space but much more time)
Joseph: If the 32-bit port supports it, you might try testing with the numbered compression options to find the "sweet spot." –fast translates to -1, while –best translates to -9; I've found that somewhere between -3 and -5 provides the best balance of compression and performance.
[I believe if you don't specify –fast or –best or any numbered option, it defaults to -5]