Ubuntu 12.04 wireless connection affects wireless speed of other devices

Many devices rotate the WPA or WPA2 group key (the key used for encrypting multicasts and broadcasts) after exactly an hour. As a diagnostic experiment, try switching to WEP or no security and see if the problem goes away. If it does, then you know the problem is likely the group key rotation interval, and you have something useful to discuss with your AP manufacturer.

Make sure you have the latest firmware on your AP. No use fighting a bug that's already been fixed.

Another thing that may be happening after an hour is DHCP lease renewal. If the DHCP server process on the AP is failing, you might not be able to get an IP address lease, which you might perceive as a failure to connect. You could try giving your client machine a static IP address that's within your router's NAT range but preferably outside its DHCP pool range, and see if that resolves your problem.

Usually on G wifi, from my experience the real speed is around 12 mbits. Here is a table with theoretical speeds:

• 802.11b - 11 Mbps (2.4GHz)
• 802.11a - 54 Mbps (5 GHz)
• 802.11g - 54 Mbps (2.4GHz)
• 802.11n - 600 Mbps (2.4GHz and 5 GHz) - 150Mbps typical for network adapters, 300Mbps, 450Mbps, and 600Mbps speeds when bonding channels with some routers
• 802.11ac - 1300 Mbps (5 GHz) - new standard that uses wider channels, QAM and spatial streams for higher throughput

Below is a breakdown of actual real-life average speeds you can expect from wireless routers within a reasonable distance, with low interference and small number of simultaneous clients:

• 802.11b - 2-3 Mbps downstream, up to 5-6 Mbps with some vendor-specific extensions.
• 802.11g - ~20 Mbps downstream
• 802.11n - 40-50 Mbps typical, varying greatly depending on configuration, whether it is mixed or N-only network, the number of bonded channels, etc. Specifying a channel, and using 40MHz channels can help achieve 70-80Mbps with some newer routers. Up to 100 Mbps achievable with more expensive commercial equipment with 8x8 arrays, gigabit ports, etc. 802.11ac - 70-100 Mbps

(Source)

Right now, I've 802.11n network @ home, but even the router and my PC says it's connected @ 300 mbit, I can get only about 20 mbits real speed through a wall.

The Wifi "standart" speeds are not achievable in typical conditions, it sounds to me more like marketing speed.