Ah, the "you can write in one context, so you must be an expert in writing in another context" fallacy. I've been on the receiving end of that too. Being a good academic writer, or engineering writer, or anything else doesn't mean you can automatically write good user-oriented material (or vice-versa). The person asking you to do this is making an unwarranted leap.
However, for the task you describe -- 250-300 word user-oriented summary -- it is probably not unreasonable for someone with some experience in user-focused writing, who is already a SME, to produce a reasonable draft in 60-90 minutes, though I think that's rather aggressive for a final product. So while you won't be able to do it that quickly to start (because you don't yet have experience with this kind of writing), it's not impossible for you to get there.
Complexity of the product is a relevant factor, but if the word-count isn't increasing with complexity, the impact is on your learning the product more than on the writing. Consider what you would write in a 250-word user-oriented summary of an automobile; while the engine and onboard computer and crash-avoidance tech and whatnot may be complex, you won't have room to get into that. If your focus is on the user then it's on capabilities, not implementation details.
To assess what kind of productivity you can achieve, you're going to need to do a few more -- and, ideally, examine some written by other people if you haven't already. If you don't yet have assignments for others, I would suggest trying a few on your own -- pick products you already know that are of comparable complexity, write summaries like those being requested, and track how long it takes you. Once you have more data you'll be able to have a more-productive conversation with the person assigning you these tasks.
(I've been doing tech writing in the software domain for 20+ years. I mostly write for programmers, not for end users. I avoid marketing as much as possible.)