The Vivante GC860 is a single-core GPU, obviously his performances can't be exciting... but it runs at 444 MHz (IIRC overclocked to 500 MHz on GCW Zero), the fillrate is more than enough for the low-resolution of the "Zero"... as Pcercuei wrote, probably the bottleneck is the memory bandwidth and the "unoptimized" code... an example can be Quake I, the current game-engine porting is troubled from several slowdowns* (this game ran perfectly with a "prehistoric" 3DFX Voodoo Graphics).
*Probably a single shader it's a "hindrance" to render a complex scene, but this should not be a problem with old games like Quake I, Quake II, Hexen II, etc...