Very Slow Bandwidth with Personal Hotspot

I'm running iOS5.0.1 on my iPhone 4S and everything is working fine besides the personal hotspot mode for which I have a specific plan with my mobile carrier (Orange France).

When I run the speedtest app from my phone, I usually get around 4Mbps as download speed but if I activate the personal hotspot, running a bandwidth test from speedtest.net website from a computer connected through wifi to my iPhone outputs 250kbps max as download speed. I have, of course, tried this with different computers (only one connected at the same time) but same results. Speed are so slow that even web browsing is unusable.

My mobile carrier is admant that the problem comes from my iPhone which I have alaready restored but that didn't change anything.

Anyone encountering the same problem?

Thanks.