Just out of curiosity, what speedtest are you using? I've just started using »www.measurementlab.net/run-ndt which seems to be quite good, can give you some useful info. What errors are you getting? Does anything pop up in the console logs on the router?
You're doing a lot with that router so it could be that it's CPU is maxed. As I said, ours will top out at ~30Mbps downstream just doing NAT - no ZBF at the moment. If it's gotta inspect all the packets coming in as well, I'd expect that to add more load. Try a
sh proc cpu history and see what the cpu loads are like. Though, saying that, I'd expect it to just get slower rather than cause a connection to drop if the cpu load was causing the problem..
I normally just use speedtest.net (37.99 down / 9.00 up)
I had a go with the site you linked and got 37 down / 9 up. I can post the details too but they dont seem amiss.
If you're just using NAT on the router what're you using for the ZBF? Another Cisco or proxy server? NAT on our router is working fine, although non-used ports are closed rather than stealthed. The speed is absolutely fine too, it's only when we enable basic FW that we run into problems.