Report on average pageload times?

I am trying to create a report on average pageload times, but with no luck :-(
I put in the JS code as per the "Piwik Web Anlytics Essentials" book and now for each pageload I have a custom variable (scope = page) which shows its load time in milliseconds. However, I cannot extract it from there in any meaningful way:

- in Visitors Log, I can see it when I hover over the page but I need to go over each separate pageview

- in Pages widget - there is no column for it (ideally, I'd like it there as an additional column)

- in Live API, I cannot get a report for it (maybe I try with wrong parameters? this is what I am trying:
?module=API&method=Live.getLastVisitsDetails&idSite=1&date=today&period=week&segment=customVariableName1=@Pageload&token_auth=***token***&format=PHP
- it returns nothing though there are page views with Pageload variable set. Ideally, I'd like here a list of pages with average value of the Pageload variable for the week.

- in Visitors / Custom variables - I see a table which says
Custom variable value 418, visits 1, unique visitors 1 etc. but there is no list of the pages where it happened, just an average over all views which doesn't help me because I want to find the slowest pages.

Any ideas?