Factors that may affect time taken for ENABLE VALIDATE (4332532)

The 'Revalidate originally validated constraints' option was not selected but during the post-switch validation it still runs and the SQL command 'ALTER TABLE xxx ENABLE VALIDATE PRIMARY KEY' took 1 hour to complete on table A but took only 1 second on table B. Table A and B has around the same number of rows but the size of table A is about 5 times the size of table B. What are the factors that may affect the time taken for ENABLE VALIDATE and is the table also locked during the validation?