What most people don't understand is that putting a malware/virus programs on Android is more to prevent virus' transferring to a computer with Windows on it. Just like if you had a Mac networking with a Windows computer, you would need a Virus program on a Mac to prevent transferring a virus to Windows.
Do you need a virus program? That question is still being debated. Most experts argue about if it is really needed for Android or not. This again is up to you to decide. Having it on your device, if you choose the right AV program, won't slow down your device. I use Lookout, which is free, and doesn't slow anything on the phone down, and does more than just being a malware protector. It has a locator and file backup to servers.