Hi Jan, KoalaSafe is 150Mb Wireless N. 150Mbs = 18.75MB/sec. So, what you're seeing is about right. That speed is typically sufficient for most uses, streaming, gaming, browsing etc.
That said, we have a higher spec device in the works. Still a few months away though.
The LAN port on the KoalaSafe can support up to 100Mbs, so if cabling is an option, that's one way to up the throughput.