Agree with Mick Kahn about using a note. As to the spinning cursor and deleting the field if you're unable to do it in the UI, the spinning cursor usually means a fatal error occurred somewhere, so check the web server logs or CiviCRM ConfigAndLog. To delete the custom field manually:
Make a database backup.
In the list of custom fields if you hover over or ...
You haven't said what field type you were using, but I think you should probably be using a note field rather than an alphanumeric. My understanding is that the note field isn't stored as fixed length.
Karin's answer is a good one. However, not all smart groups are created equal! Culling the right smart groups is what will fix your deadlocks.
Identify your slow smart groups. Here's a script (rough, could use improvement) that you can run that will show you how long it takes to build each smart group. The latest version is available on my Github:
We have seen this on large instances. Things to do:
upgrade to 5.13.5 (this also puts you close to the next ESR - a paid for service - which will be released on Nov20).
There have been performance improvements - this will help.
cull your smart groups; do you really still need all of them.
reschedule the cron job -> ask yourself how up to date your smart ...