How efficient is the immigration process for Non-O retirement extensions in Thailand?

Same for Phuket. If you have all documents required from the list and are well clothed, you don‘t have any issues at all.

Never had a problem in Phetchabun 30 minutes in and out, very friendly.

We are all lucky because we are due in low season. I can report the same good experience at the Phuket immigration office. Did my yearly Non O (retirement) extension and was in and out within 20 minutes. The only difference was that I had to collect my passport the next day.

Same in CM if you make an appt online. Last time I was in and out in 25 minutes with a new Non-Imm O extension.

Chiang Rai is a great place friendly polite people with few tourists

I had a very similar experience today at Hua Hin IO. I arrived at about 11:40 and by the time I had completed the form and had everything checked it was 11:58, I thought there was no chance of me being seen before lunch, but decided I would get a number to try and be first after lunch. I was amazed to be called over immediately and I filled out my re-entry application whilst the IO checked and processed my extension. 20 minutes later I walked out with a years extension and re-entry permit. All the staff were super friendly and the officer even answered some questions I had, as I was moving from the 800,000 to the monthly 65,000 baht method. Brilliant service as always 👏👏👏