This may be a very easy interview if you have the specific experience tested. HubSpot only appears to ask candidates two main questions, with the same three to four questions repeated in other comments. The questions test your experience in basic application design, some experience with REST, RDBMS, and optimization.

For optimization, the questions require high-level answers, like proposing the implementation of a cache, as opposed to time/space complexity questions. Giving the appropriate answer to both these main questions quickly seems to be by far the most important aspect of the interview. I had not prepared well for other questions on my job experience, but the feedback I received focused only on the two whiteboard questions. As I said, the other questions don't seem to be very important, but I'd recommend preparing some likely answers using the STAR technique. I did this after my interview and was able to come up with much better answers than the ones I gave at the time.

I was hinted at by the recruiter to research the answers to the questions they ask. In hindsight, I wish I had done this better, as the questions are relatively simple, but you are given little time or scope to improvise during the interview.

There is also a pre-interview test. I won't write much about here; the details are covered in several of the other comments. That test appears to be mainly about how fast you can finish. The code I ended up submitting for this was nothing to be proud of, but it was submitted fast enough to get me to the next phase.

What technologies have you incorporated within a team?

Tell me about a time you were behind on a deadline.

How would you handle having a proposal for a design or new technology rejected?

Whiteboard: Design a URL shortener API, with metrics capturing and some optimization suggestions.

Whiteboard: Design a data model for storing users, friend lists, and posts (small messages). Design an API for fetching a paged list of time-ordered posts from a user's friends (like a lightweight Twitter).