CPU meter accuracy?

What it meassures, single core, all the cores?
What is more reliable the simple meter in the main interface or the one that opens when pussed?

One core. More precisely the CPU cake diagram measure the time it takes to process a plugin in proportion to real time, in average. The light-red part of the simple meter should show the same proportion - total CPU spent of all plugins, in average. The darker-red part of the simple meter show the peak CPU usage. - i.e worst case encountered within the last second

Thank you, its more clear now.
With today multi core cpus (my new Iphone XR has 6!) seems there is spare cpu to load synths and guitar processors.
My XR is my first device with IOS12 but strangely I cant use my Line6 Sonic Port anymore (not enough power apparently) but I will probably ask this in another thread to see if others have encountered problems powering usb audio devices with IOS devices loaded with IOS12…