If you create a network that is b/g/n compatible, then yes you will always be running at half speed. And I think it is 130, so it is more than half.
There is a way around this. I have the Airport Extreme base and wanted to get full n speeds, but I also have a wife who has a Powerbook that is not n compatible, and also an iPhone that needs a g network... So, I set the Airport Extreme base to be an n only network, getting full 300 speed, and I connected my airport express (b/g network) hardwired to the extreme base. I set the slower router to be in bridge mode and the faster router is handling all ip addressing. It works perfectly.
Something that I was surprised to find out with this setup is that my wife, connected to the g network, can access the air disks that are on the n router. I can even print to the g network printer even though I am on the n network. I can also access the airtunes speakers on the slower network even though I am on the n network.
I hope that makes sense.