Currently there are no public info available concerning Chromecast device CPU Marvell 88DE3005. We can look inside of its brother 88DE3100 to get first estimations of Chromecast SoC computation and graphics performance.
Marvell 88DE3100Block diagram
Let’s assume that 88DE3005 is single core:
In this case, power of 88DE30005 is 2.61 DMIPS/Mhz, resulting in 3000 DMIPS when clocked at 1.2 GHz (note that maximum operational frequency of 88DE3100 is about 1.6 GHz).
This figure (raw performance) can be compared with mainstream smartphones, but it is too low when compared with any modern tablet.
Concerning GPU, Vivante GC1000 is member of low-end family and cannot compete with Mali-400MP4 and newer GPUs. Even its compatibility for 3D games was questionable – this is expected to became better as Google adopted this GPU.