Check out these:
Bad Wifi on the N950 -- no reason why it shouldn't work on N9; also touches on IPv6 connectivity issues
Linux iwlagn problems

i have the same issue. And when running nitdroid the wifi performance actually is much better. Both distance and dl speed. Never had this issue prior pr1.3.

at work we have the same internet provider but the router only running in b mode and its it much faster .

going to do some test at home.

not related but, can you run wifi hotspot/tethering on nitdroid?

no its not working atm.

The following in Steve Ballmer's voice when comparing the difference between NiTDroid and Harmattam's WiFi performance:

“Drivers, Drivers, Drivers!”

it Has a power saving loop, you can disable it via n9tweak

i get error when using n9tweak:
arning! This app could potentially bork your system

Select an option of your desire (Q to quit): Ps

This will run a loop every minute
and keep wifi powersave off.

This is very power hungry.

Press enter to start (ctrl+c to end):
Password:
sh: /sbin/iwconfig: not found
sh: /sbin/iwlist: not found
sh: /sbin/iwlist: not found

I haven't used it from n9tweak yet but you can use GConfik to edit the gconf value...

The gconf value you are intersted in:

Default value is 600(seconds)...Change it to a high value...

I went to router and set multicasting to off. Dl speed are now greatly increased even when running at n mode on router.

will test more but it seems to work much better.

Edit:
After doing some test with it on and off the results are the same.
However now wifi works good again. Dunno why.

Thank you for supposing some handy links, but I haven't solved my problem yet.
I went to the terminal, then I connected to my wireless N network, and after that I executed 'develsh', 'iwconfig wlan0 power off' commands and of course I verified power saving mode status by viewing 'iwconfig' command's output. It said power saving mode was disabled. Then I tried to download a large file from the home network again. Disabling power saving mode made no difference.
Okay, I also blacklisted ipv6, rebooted my phone and redone all measurments - it gave no result.
So I conclude there's no need in using some kind of 'tweak' to make the power saving mode prohibition permanent, cause such a measure doesn't take effect at all.
Moreover, I've read articles which had been suggested, and I can say I have no problems with authentification/deauthentification. My connection is stable, I haven't noticed any kind of interruptions or ping loss. My problem is that enabling Wi-Fi N mode on my router reduces connection speed half instead of boosting it twice.

In one of the suggested articles the author said the problem was in the driver and suggested reloading the driver with N mode support disabled. But you see - it's not a desired solution. The aim is to get N mode work perfectly. So.. maybe somebody can suggest using another driver?

I can say that I'm disappointed in Wi-Fi hotspot/tethering N9 function too, because it doesn't create normal AP (Master Mode), it just uses Ad-Hoc mode. And also there it's impossible to use WPA/WPA2 encryption with 'WiFi Hotspot' app.