This is quite different than capturing the load time in a dimension, so I thought I would touch upon how to do this in Adobe Analytics (which can also be done in Google Analytics). If you want to store page load time as a metric in Adobe Analytics, you would pass the actual load time (in seconds or milliseconds) to a Numeric Success Event. This would create an aggregated page load time metric that is increased with every website page view. This new metric can be divided by page views or you can set a separate counter page load denominator success event (if you are not going to track page load time on every page). Here is what you might see if you set the page load time and denominator metrics in the debugger:
In this case, there is a calculated metric that is dividing the aggregated page load time by the denominator to see an average page load time for each page. There are also ways that you can use Visit metrics to see the average page load time per visit.