Your steps to shrink the log file are extremely dangerous with potential risk of losing the db completely. No one should do that.

You could diff backup db, set it to simple recovery, shrink the log file, set db back to full recovery, diff backup db.

Instant file initialization works only on data files, not on log files.

See which windows process is taking CPU. See what SQL sessions are doing (sp_whoisactive or sp_who2).