Oh ... Windows ... not a windows person ... but ...
From what you've described it doesn't sound like your cron job wasn't set to run frequently enough ... even when site was a 3.6 (prior to upgrade). Once a minute is recommendation.
Adhoc task can be run from command line ... suggest you do ... located in admin/tool/task/cli/
Will get the help screen for the script
Scheduled cron tasks.
--showsql Show sql queries before they are executed
--showdebugging Show developer level debugging information
--execute Run all queued adhoc tasks
-h, --help Print out this help
$sudo -u www-data /usr/bin/php admin/tool/task/cli/adhoc_task.php --execute
you might have to let it run or run it multiple times to catch up.