Here is my experience. I used Rails for one project of mine that requires processing of millions of data rows. Because the Rails ORM create an object for each data row, we end up using a lot of memory. We had to get a 2GB memory server to hold up the project. Even after we avoided the ORM (which removes the pleasure of coding in Rails), the memory usage was still high.

Sure, we could process the data rows outside of Rails in C but because the processing of data rows is an integral part of the project, that would mean coding 80% in C and 20% in rails. Not exactly an enjoyable experience.

So we rewrote it in PHP and avoid objects and use just functions and hashes/arrays. And it worked very well for us. The site render time drops from 0.8s to 0.03s. Memory usage rarely exceeds 100MB.