So, looking at it, it looks like this particular person does typically 1-3 new designs a day plus various minor variants - sometimes more, sometimes less. Unusually for people on sites like this, it looks like they've earned enough that they could be treating the site like a full time job.
I've known a few designers who work at a similar (often faster) rate: working for agencies at the low cost, high turnover end of the market.
They're good designers, reasonably well paid, and their speciality is essentially mastering a few in-fashion styles and bashing stuff out in those styles at speed: basically the service is "this month's web design fashions in your size and colours, fast!".
The impression I've got is that they bash out 1-4 fresh design concepts a day, plus minor alterations, meaning something like 3-12 actual comps/knockups a day.
It only works if all the below are true:
- They just make a simple homepage knockup: no code and no real consideration of content, information architecture, responsivity, etc. Usually they work for an agency who do then implement the design, and there will be a separate team of developers and IA/CMS site builders who work to the signed-off image: the designer's role is getting from pitch to comp to agreed, signed-off concept as quickly as possible, then they pretty much forget about that site.
- They are simply picking an established fashion and applying it. There's not much creativity or deep thinking: of the people I've known who work like this, maybe half like it and treat it as a "It pays the bills and I'm good at it" regular job, the other half feel stifled and are trying to find other work where they have room to breathe and feel like a designer not a machine.
- The clients have basic expectations: they just want a professional-looking website that does the job. The products are rather off-the-peg, and there isn't much room for any deep consideration of the clients' needs or for many revisions processes.
- The designers do this a lot for very similar things. It seems to take a couple of months of work in this kind of environment to become real design-churning robots, starting out at 0.5-2 a day - and of course it's an environment that suits some but really doesn't suit many. It has to be fairly monotonous to build up the pace. I couldn't work on virtual autopilot like that, I'd go mad.
- There's a good infrastructure of resources/assets ready to drop in place: ready-to-use stock images, fonts, textures, fonts, decorative elements, button designs, etc plus some kind of communication or rotation to make sure the same ones don't get over-used.
So, in that kind of environment, it is possible to work at this pace. The key questions are: a) Do you really want to do that kind of work? b) Does that kind of service suit the clients you have access to?
For crowdsourcing/spec-work/design-contest sites, it's really the only approach that makes any sense, but those sites usually don't make financial sense anyway except for a few serious full-on design robots: see Why was my experience with a logo contest so miserable?.