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My iMac 27" (late 2009, 10.8.2) wifi (3-4 bars) speed doesn't compare to other computers at home. The same Youtube video in 720p loads without delay on my dads computer, however on my iMac (I usually just watch 360p or 240p) because it's so slow.

Now I messed a lot with network preferences when setting up servers and stuff so these speed issues are probably my fault. Strangely Speedtest.net shows the same results as any other computer on the network, but youtube just doesn't work well.

Are there any things I should try? Is there any way to restore defaults? I wasn't able to find the "/Library/Preferences/SystemConfiguration/preferences.plist" as mentioned on some blogs.

I'm also considering reinstalling Mountain Lion cause I have some issues with loud disk activity even when I'm not using the computer. Would this delete all my files and apps? Would it fix the wifi problems I have? How much changes does it do when I install it on my HDD that already has Mountain Lion installed.

All answers will be greatly appreciated.

- Wifi info:
PHY Mode: 802.11n
BSSID 00:17:3f:3b:64:7c
Channel: 3 (2.4GHz)
Security: WPA Personal
RSSI: -55
Transmit Rate: 54
MCS Index: 0
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Did you try to put your Mac into the same location as the PC to compare the speed? Did you move the base station to see whether this makes a difference? There are a lot of things influencing a WiFi signal. Alternatively you could try to boot from the Recovery Partition/DVD and check speed from there. –  patrix Feb 3 '13 at 15:10
The computers are in different rooms. Both of them have full bars on connection. I forgot to mention that I use to watch 1080p youtube videos on the imac without a problem some time ago. –  Matthew Abrman Feb 3 '13 at 15:14
What browser are you using to watch Youtube videos? If it's just Safari have you tried alternate browsers (i.e. Google Chrome or Mozilla Firefox)? –  Mort Feb 3 '13 at 15:21
I normally use chrome, just tried safari, same results. –  Matthew Abrman Feb 3 '13 at 15:23
Simple question, does your rate goes up if you disable other computers. Your MCS Index shows zero ? –  Buscar웃 Apr 9 '13 at 17:10

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