Any way to show render times of widgets to aid performance debugging

You can get a lot of these details by doing a trace from Administration > Monitoring > Tracing. Start a trace, then load the page in another tab. You'll get a breakdown of requests made (including scripted file requests/renderings) and can drill in for additional details on requests.

This sounds perfect.. however, I've just tried it & I'm unable to view the results.. the page is hanging when I click on 'Details'

Does it store the result anywhere other than the browser.. somewhere less susceptible to issues rendering a large number of actions?

Hmm, Firefox seems a little more tolerant.. it's Chrome which locks up for me.

Tracing data is sent directly via the socket bus to the browser. Do you have a Socket Bus installed, and if so, which one/how is it configured?

As a stopgap to view the data, you could register a client socket script to listen for tracing socket messages and log them to the console / do something else with them.

$.telligent.evolution.sockets.tracing.on('tracing.step_completed', function (data) {
	// ...
})

Note that the socket message name 'tracing.step_completed' is not considered extensible or upgrade-safe.

We're using an Azure Message Bus. It's configured as per the setup guide ;) 

I'm fine with using Firefox it that's a little more stable than Chrome for receiving a large # of actions.

The JS for the tracing looks useful.. thanks for that.