you have to upload the scans, then pay. Then they send you a link to book an appointment, you go, check details and collect your paperwork. Took about 15 mins in the actual embassy. (AIA Tower, next to St.Louis BTS station).
"The processing time for visa applications through the e-Visa system will take approximately 10-15 working days (if all required documents are complete), from the date the visa fee is paid.", on the website for all to see.