I have been using mrxvt for years. It's also "multi-tabbed". Currently,
I'm running 10 terminals in a single mrxvt process and it is currently
using Size: 2644K, and Res: 4472K and barely touches the CPU. I'm
running a custom compiled version with all the fancy features turned
off. It is extremely fast and reliable; I haven't found anything faster.
-pachl