might be worth reading (more the later specimens, than the old apu series with ancient AMD Jaguar cores); e.g. a baytrail-d Atom j1900 can do ~830 MBit/s max. with sqm/cake and all bells and whistles (or the full nine yards (easily) without sqm).
New 4-port (up to 2.5 GBit/s) n95/ n100 mini PCs start around 130-250 EUR when ordered directly from China, OpenWrt should do (does) a pretty good job on those.