Poor WiFi Speed on iMac but excellent with iPhone 4S

Hi everyone

Hope someone can give me some ideas on correcting my internet issues...

I've recently upgraded my internet speeds through my ISP to have 50Mbps download and 2Mbps upload. I use my 2TB Time Capsule as a router as it has by far provided the best connectivity than any other router. I run dual network - connect my iPhone 4S through standard network and my iMac through 5GHz network. Since upgrading my speed, I have noticed that my iMac speeds vary greatly, but most of the time I'm struggling to get more than 10Mbps download, however on my iPhone I easily achieve almost 40Mbps. I have tried different browsers, and ensured all other applications are closed and even switched the iMac (2009 i7 quad core) to the standard network. At that point I noticed a slight increase in speed but it was short lived.

I am stumped on what to try next. Does anyone have ideas? The iMac is a little hot, but nothing that isn't normal so I can't put it down to heat or anything. Is there anything I have failed to check that may help?

I recently installed Windows 7 through bootcamp but didn't expect that to impact anything, however it was around a SIMILIAR time that I noticed issues, as previously I was getting a consistent connection of 20Mbps before I upgraded.

Any advice would be excellent!

Thanks everyone 🙂