Ok, that makes more sense now. You've got a wired connection, from ISP modem, to primary router (wired), then to secondary router (wired), then to PC (wired).
In that case, yes, it is kind of weird to be losing out on 10Mbit/sec of your internet connection speed. At those speeds (less than 100Mbit/sec), all of your routers in the chain, should be capable of routing full ISP speeds. Do you have the two routers connected LAN-to-LAN (in that case, it would only really be testing the WAN-to-LAN of the primary router, and the router's switch ports on the second router)? If so, then something is flaky with the second router.
Edit: Upon reading your OP, I see that you've got your ISP plugged into your primary router's WAN port, and then your secondary router's WAN port is plugged into the LAN port of the primary router. Which is setting you up for double-NAT, which is not a preferred arrangement, and possibly part of the explanation for the slowdown that you're seeing.
How old is this Zyxel router? Is it a 'G', 'N', or 'AC' router?
Regardless, if you just want to use it as a wireless AP, then you need to connect it LAN-to-LAN to the primary router, and then disable DHCP and NAT on the Zyxel.
If it's older than AC, I recommend replacing it with a newer (even AC750 or AC1200) router.
My preference is for Asus routers, either an AC66U or AC1750 (same H/W), or newer. I don't like to mess with anything slower. (Unless it's for a grandmother that only checks e-mail and surfs the web. But what if their grandkids come over? That's one reason why I try to stick with AC 2x2 routers at a minimum these days. They aren't expensive, $40 or so for one.)