Whether its a pneumatic tire or a solid tire a harder tread compound gives lower rolling resistance.
From page 40
http://www.geax.com/vedit/18/immagin...2009_EN_LR.pdf
" Sticky Soft compound on side knobs and ARAMID RACING 3D on the central ones. That's the best compromise you can get from both the compounds: cornering grip and braking performances keeping low the rolling resistance. Soft side knobs guarantee a strong bite in turns and excellent grip in all situations whilst the central (harder) compounds with aramidic particels makes for lower rolling resistance reducing wear and tear to acceptable levels. It's the ideal solution for racing tyres"
Note on this page that all the low rolling resistance tires have a hard compound
MAXXIS TYRE TECHNOLOGY