Hangfire currently cannot run a job every 10 seconds, the minimal resolution is 1 minute. The less the resolution, the more performance hit would be.
If you have an always running job, you don't actually need a job scheduler for that: just start a separate thread and throttle it to whatever resolution you like. This way there won't be any overhead of constant database polling for new jobs.
In terms of memory, the hosting process doesn't really matter. If your job consumes 100Mb of memory, it would add 100Mb regardless of running in IIS or in a separate process. Bare IIS worker process is not much different from any other process, and would definitely pale in comparison to a memory used by your code.