That's what I have been told -- so long as you are armed with the appropriate application form, acceptable photo affixed and six attachments -- and it is not crowded. Some difficulty/delay has apparently been encountered in the past with poor CNX staffing, somewhat confused and/or crowded conditions and/or the need for new photocopies and the lack of 2d floor services to get photocopies at least at some hours. Maybe all that has been rectified. Chiangmai Immigration leadership and processing have certainly improved.
The easiest path, really, if feasible, is to get a multiple entry re-entry permit at each annual visa extension time....unless you invest in a ThaiElite visa.
Processing normally does indeed take only about 15 minutes including a photo taken.