Another clue. Don't know if it's related but it's suspicious since it involves reading tags.
MC has for several years struggled to correctly identity the compression type for some video files.
Today I imported a 1080p x265 mkv file and MC reported the compression as "mkv video (video: HEVC, audio: LAV AAC ADTS)".
I knew from experience that this was wrong so I clicked "Update Library (from tags)" to refresh the compression detection and MC correctly reported "mkv video (video:hevc, audio: aac)".
The same file caused a 30 second freeze on import (as described above), and the "Update Library (from tags)" operation also (sometimes but not always) caused a 30 second freeze and so I wondered if the two problems are related.