Using encryption on my Wi-Fi router really slows things down. If I restrict Wi-Fi access by MAC address, so that only my portable can connect to it, can I then turn off encryption?

No. While limiting access to your Wi-Fi net by MAC address is a good idea (every computing device has a unique MAC address, usually printed on a label on the bottom along with the serial number), there are two big problems with relying on this security method alone. First of all, since Wi-Fi signals are simply radio waves, they can be read by any receiver without needing to be connected to your router. So if you don't use encryption, the data you transmit and receive wirelessly is sent in plain text for all to read.

Second, MAC addresses can be "spoofed." That is, someone can read your MAC address as it is sent over the air, and then pretend to be you, connect to your router, and gain full access to your network. It's always best to use every layer of security available to you, from system-level approaches like personal firewalls and antivirus software, to router-based functions like encryption, closed networks, and MAC address limitation.

So you should always use encryption, preferably WPA or WPA2, the latest and strongest. The trait that makes both WPA and WPA2 much more secure than WEP is the fact that the encryption key changes with every session, and keys are also specific to each client on the network. While WPA (Wi-Fi Protected Access) was a big step up from Wi-Fi's original WEP encryption protocol, it was always meant to be an interim solution until the government-grade IEEE 802.11i security standard became final, in the form of WPA2. Don't buy new Wi-Fi gear without it.