I'm very aware of this, but that's not the problem I'm talking about. The Twitter Fail whale doesn't happens because of front-end problems, nor Facebook's slown

I'm very aware of this, but that's not the problem I'm talking about. The Twitter Fail whale doesn't happens because of front-end problems, nor Facebook's slowness, nor the cpu overload you see on your thumbnail processing machines, nor the mem crashes you experience when memcache servers run out of memory... I do agree with you that a lot of what users experience as slowness IS front-end stuff. BUT there is also a lot of issues with the backend not working as expected. Lets say, front-end optimization is like shooting the low hanging fruit :) Nevertheless you're right I should have commented it somewhere.