What advice would you give to someone who's getting a slow response?
For smaller companies, they most likely don't have the resources to keep in touch with everyone. For me, this is the main value of an Applicant Tracking System: making sure no one falls through the cracks. That being said, most small companies don't have them, so they are relying on their memory + spreadsheet to carry them through.
If you're working through an external recruiter: talk to them, they have every incentive to push you through the pipeline and have a direct read on the process.
If you have a contact in the company or you met with someone in person: send them a quick follow up email is enough to move the pipeline usually.
If you've blind applied: then I would suggest sending them a quick note to get back onto their priority queue. Whomever is in charge of the process WANTS to respond to everyone, but if they don't see a fresh email sitting in their inbox, they quite possibly have forgotten that you're in the pipeline.
Also as mentioned below, pursuing many job opportunities in parallel is good for you and can be a great forcing-function/bargaining chip in late-stage hiring processes.