After reading all the responses to this, I was curious to see how it would do on my system. It seemed to do fine. 

wayne$ ruby largearray.rb 
6.730365

But this is also on a 64-bit system with lots of RAM. So, I'm not sure it's a fair assessment of your script.  And normally I wouldn't create such a large array. Now, maybe you were doing this to test some larger script and wanted to see the performance, but I've got some pretty large arrays in a few of my scripts and they don't have performance problems on any of the computers I use them on. But I don't think they come close to the size you created here. 

Wayne