Try connecting to your router with an Ethernet cable. Turn off Wi-Fi. Still slow? If not, see below under "Symptom: The Wi-Fi network seems slow":
Hi, i have a one month old iMac 2.7GHz intel core i5 running 10.8.2. It was running perfectly until about a few days ago when i noticed that the internet speeds were impossible. Ran speed test and found that I was getting 0.4mbps vs 5mbps that I normally would get. I have a macbook air and 2 ipads wifi running on the same home network that are all fine. i have read a lot of non conclusive posts about what might be the issue but non of them resolved my issue. I am really puzzled as to why the sudden slow down. the iMac is hardly used - only for downloads and web browsing. Would appreciate any help on this. Thank you!
Thanks for the response Linc. As my router was in the adjacent room, I had to unplug the iMac and carry it over to try the ethernet cable connection. But before i did that, i decided to try the wifi speed again whilst physically next to the router. suddenly the speeds are back to normal again. strange. tried with ethernet and its fine too. so decided to move iMac back to study and monitor the situation for awhile. I'm unsure as to why this has happened. Since this is a new machine, I'm covered for a year should there be a hardware issue. Many thanks again for replying.