All agencies do that. The time to reply is the biggest tell, since DM interfaces tends to be notoriously bad regardless of site. Like imagine talking to 100+ people non-stop on discord/telegram, and these are sites fine-tuned for talking through DMs.
If you want to manage DMs fast "alone", you'd have to make a custom frontend and even maybe a middleware backend for automation and advanced filtering, or you hire a bunch of guys to do that. The probability of an onlyfans model going for the former is approaching 0%.