If you have been collecting performance data with Matomo since before version 4, you will have collected average generation time instead of average page load time data. This is currently in the process of being deprecated but in the meantime, you can still access your old reports within the Visits > Overview and in the future it will still be available from the Matomo Reporting API.
The simplest way to get your average generation time data for your site is in the Overview page of the Visitors section, accessed via the main navigation menu down the left-hand side of the screen.
On this page there is a Visits Overview report which shows several high level metrics for your site, including visits, actions and most relevant now, your average generation time.
You can also click on the label for this metric to load the evolution graph at the top of the page, so you can see how it has increased or decreased over time within the range currently selected by the Matomo date selector.