I'm experiencing the exact same thing. When I installed Mint 13 Maya (64-bit), downloaded Oct 12 from the official source, I thought my computer had broken, because the installation took +24 hours. Literally and seriously. Because I connected to my WLAN in order to pull down the latest versions while installing....
Today, I ran a speed-test using WLAN, which resulted in 156 kb/s downstream. The upstream failed due to timeout. I plugged in the cable - connecting it directly to the same wireless router - and re-ran the test. This time I got 16.98 Mb/s down, 1.02 Mb/s up, and 23 ms ping.
To test this, I booted the very same computer using an Ubuntu 11.04 CD that I had lying around, and ran a Wireless test achieving the same result as with Mint via the cable.
Thus I conclude that the issue is not with my router, not with my hardware, but with Linux Mint.
Funny thing is, in Ubuntu when connected to my WLAN, it says it has a 54 Mb/s connection when the same in Mint says 1 Mb/s.