Launch specific device simulator How can I launch the device simulator on a Mac and specify which specific device I want, for example an iPhone 6s or an iPhone X, without having to launch Xcode (which takes forever on my old Mac)?
You can use simctl command from xcrun. Here's how to do it:

$ xcrun simctl list


Sample output:

(...)
== Devices ==
-- iOS 12.2 --
    iPhone X (AC5DA9BB-2198-4E6B-9CFC-A407730FF853) (Shutdown) 
    iPhone Xs (69A34565-5621-419C-8F37-3089FEFA52C1) (Shutdown) 
    iPhone Xs Max (8192F2B2-3F3F-49C1-AC36-D88550D48FCA) (Shutdown) 
(...)


The string in braces is UUID of the simulator device.

Boot the device with:

$ xcrun simctl boot $UUID


Now launch the simulator with:

$ open /Applications/Xcode.app/Contents/Developer/Applications/Simulator.app/


If you do not need to switch the Simulator type, just run the last command.

More info about simctl and how to use it see the NSHipster article.