Lenovo's "
Think disad
Vantage" suite was removed out of interest since I don't use any of the features and the problem is solved. Speaking to colleagues reveals similar incidences of trouble, and the specific culprit is believed to be Access Connections (a utility that tries to be clever and manage all network connections, remembering what the appropriate default printer should be depending where you are, and so on).
Anyway, bloatware removed and I've got my fast machine back.
Cheers anyway, and especially thanks for the link to Process Explorer; handy to know for future reference.