There's been a few post on AC about mobile security over the past year. You can search for them and read up.
The short version is, as long as you're very careful, you don't need one. So if you only download known and trusted apps from the Play Store (or Amazon App Store or other trusted market), read reviews to see if others have had problems, and you don't download and install stuff from random links (there was one going around facebook recently), or the "seedier" side of the internet, then you don't really need virus/malware security. If you do download apps from random sites, or install pirated apps, etc, then you're putting yourself at risk.
A couple of other notes:
1) Just like on a PC, I wouldn't run more than one security app. Generally they don't play nice together.
2) Supposedly the slowdown is very minimal. On the order of a second or 2 when booting and fractions of a second when downloading.
3) Many security apps provide other useful services like locating your phone if lost or stolen, backing up files and contacts, and giving you the option to remote wipe to protect your data if your phone does get lost or stolen. These services are much more valuable and useful to the average person than the antivirus stuff.
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