According to the Z5 datasheet »
ftp://
ftp.zyxel.com/ZyWALL_5_U ··· TM_2.pdfthe Firewall Throughput is 65 Mbps. This is both ways, so one way max. speed would be 32.5 Mbps. So yes, you're maxing out the unit.
If you want to look at next generation ZyWalls check this »
USG series FW 3.00 Comparison