I often find this at the bottom of pages.

Page generated in 0.00013899803161621

Sometimes, I come across

Page generated in 9.0122222900391E-5

What does that time mean?
I tried searching Wikipedia for E and maths but found the e mathematical constant. My guess is E stand for Exponential and -5 is the power it is raised to. And the displayed time is a really small number. But that doesn't make sense when compared to the other time in the question. 0.00013899803161621 is bigger than 9.0122222900391E-5.
If it means x times $10^{-5}$, then 9.0122222900391E-5 will be 0.000090122222900391 which is smaller than 0.00013899803161621. What does E stand for?

It means $9.0122222900391\times 10^{-5}$. This is known as "scientific notation".