According to rumors reported by VG247, the April system update for the Wii U may have increased the console’s GPU and CPU speed. The rumor originated on the TV Tropes website, from a poster who says that clock speeds have increased, but no evidence has been given to support this. According to this poster, the CPU clocked at 1.24GHz before the update, but at 3.24GHz afterward. The GPU is said to have clocked at 550MHz before the update and 880MHz afterwards.
Since no evidence for this has been provided by the original poster, the claims should be taken with a grain of salt until it can be proved. Still, there’s no denying that the Wii U is certainly speedier after the update. Games start much quicker, the Wii U interface is speedier, and loading times for games have been improved.