By default, Google Analytics uses only 1% of your site visitors data to measure site speed.
If you have a relative small number of visitors to your site, such as 100.000 or fewer, you need to adjust the sampling to a larger rate to get enough data into your reports.
To do that, you need to make a small change to your Analytics tracking code. Once you have done that, your will be able to get speed data in your reports.
The process is easier than it sounds, here are the steps: