Same exact problem, 2011 Macbook Pro with Mavericks, 16 gigs RAM and over 100 gigs HD space left. An iPhone, iPad and Macbook Air (still running Lion) have expected speeds when comparing #s via speedtest.net but my download speed on the same wireless network is literally 1/6th to 1/10th of theirs (and very inconsistent one moment to the next) - sometimes slower than even its own upload speed. Ping and upload speeds don't seem to be affected. This is the only machine at this residence upgraded to Mavericks, and it is just terrible for a platform that insists on such reliance to the internet.
I did not have this issue prior to upgrading to Mavericks and did not notice it immediately after that (but don't generally test speeds until I suspect a problem). Have tried rebooting machine, starting in safe mode, repairing perms, testing speed while logged in as Guest, turned Airport off and on, renewed DHCP lease, switching IPv6 to local-link (there is no "disabled" option in Mavericks) then back to automatic, etc. Lack of speed does not appear to relate at all to how busy the network or internet connection is by others.
Potentially mitigating factors:
This is an upgrade of Mavericks over Mountain Lion (which was a fresh install on the current HD after the old one died). We used to have a network of several older Airport Expresses; now a single router from U-Verse covers the entire area. The wireless network is closed, 802.11g, 2.4GHz, WPA2 personal security and same network name as the now-unplugged Airport Expresses (which had been WPS). A time capsule is plugged into the router and visible but set to not broadcast its own network. An AppleTV is also on this network and works fine. The router does note 22 (and only 5 via ethernet) unique devices connected to the network with perhaps 2/3 of them active at any given time.