Maintenance Mode in SharePoint Online and Modern pages

Turns out, if you append ?maintenancemode=true of your SharePoint page you will see a special maintenance view:

On your page, you will see a summary of data from the Manifest and Data tabs. The summary information includes:

Alias	The name of the web part
Id	The unique ID of the web part
Instance Id	The ID of a specific instance of a web part (that is, if you have two more of the same web parts on a page, they will each have the same web part ID, but a different instance ID.
IsInternal	Indicates whether the web part was made by Microsoft or a third party. If True, it is made by Microsoft. If False, it is made by a third party.
Version	The version number of the web part.
Environment	Environment: Indicates the SharePoint environment in use. 0 = Test environment 1 = Local Workbench 2 = SharePoint 3 = Classic SharePoint
UserAgent	A string that contains information about the device and software in use (such as browser type and version).

Microsoft article "Open and use the web part maintenance page"

SharePoint Online Performance. Easy Way to Capture Metrics

I've stumbled across an interesting Chrome Extension Page Diagnostics for SharePoint Chrome extension. It shows some useful info on the page performance.


Microsoft Article "Use the Page Diagnostics tool for SharePoint Online"

In my case SPRequestDuration and SPIISLatency were not displayed, but you can get these values by simply pressing F12 and typing g_iisLatency and/or g_duration in the console:
g_iisLatency - Shows network latency between the client browser and the IIS server
g_duration - Shows the time it took the page to be returned to the client browser

Update:
I've found out that g_iisLatency and g_duration only show up in the classic pages.