In the past I have found the embassy in Paris to be efficient for normal tourist visas, but got refused for my DTV with no explanation. I then flew to SGN to apply, it wasn't fast, as they were shut for public holidays during my application, but I was successfully in the end, all in all, it took ten working days. During that time, they requested extra paperwork, which reset the claimed 5-day process. As an aside, I loved my time in Ho Chi Minh City.
It took me almost 2-hours at Bangkok Bank in Pattaya even though I arrived before they opened. People with agent assistance appeared to get preferential service, but the bank was also incredibly busy. Think that I must have signed my name about 20 times, but left with an ATM card and passbook.
I used HCMC. Stayed in District 1, in Japan Town area which was great, it's away from the westerners tourist area, but still easy to access if you want a wild night. I found Saigon to be cheap for accommodation, meals and beer cost peanuts. After submitting my documents online, I was asked for proof of entry stamp, it took over ten days in all, but I applied during TET.
It's not just Thai girlfriends that get denied. My Belarusian girlfriend was also denied although she is an architect who owns her own home. No reason given and no option to appeal. She visits me in Thailand without issue. American immigration are full of themselves, they are convinced everyone wishes to live there and will not leave.
I am in and out a lot. Typically two months in Thailand then a month in Vietnam. Then repeat before returning to France for the European summer. I have to renew the Non O extension of stay, commonly called a retirement visa each year.