After upgrade to High Sierra, "lock" system operation is very slow.
I've seen some information that other tools are having issues as well due to "enhancements" in High Sierra to certains aspects of the system, such as lock screen, screensaver and kernel extensions.
Anyways, the Ctrl+Cmd+Q global shortcut introduced in High Sierra is blazing fast, while the "Lock" System command in Alfred 3.5.1 is slow (takes several seconds).
My immediate workaround is having a workflow that binds to "lock" keyword and triggers apple script that tells System Events a Ctril+Cmd+Q key combination has been used. This works like a charm but would be great if "native" lock command implementation was fast on High Sierra, just like it was in Sierra.