It's late but I hope this helps (and I hope others can chime in)
I second noahc's comment on the specific stack. But more generally, some keywords to help sort/pick through to optimize for 'framework that is very common'.
Ruby on Rails (RoR), Laravel (PHP) and Django (Python) is (I think) the most familiar and widely used. By default will tackle most of your needs and be familiar enough to have >90% problems you faced tackled by others (and solved by googling). Make sure you insist to use Relational Database to save headaches along the way (PostgreSQL/Postgre and MySQL is the two most common ones).
As you are going to a dev shop, make sure that they are sticking to at least the above constraints. By having those, you can shop around for other dev shops in case you need to expand/mismatch down the line.
For now, your focus as a founder is to get the main product off the ground, and performance is second to that. It's a "good problem to have" and hopefully you'll have enough resources with the product success to improve it. Generally you can throw money at scale as long as you are not as big as 100k MAU, by throwing in more server/bigger servers.
For the specific dev shop, troy's comment is great to pick one. But for the framework, I recommend leaning to the ones I mentioned.
Welcome. I've been involved in this position before (came in as a PM/Tech in a startup which outsourced the development). It went well, but would be easier if the vendor were to stick with those requirements to ease maintenance/feature development down the road.
I second noahc's comment on the specific stack. But more generally, some keywords to help sort/pick through to optimize for 'framework that is very common'.
Ruby on Rails (RoR), Laravel (PHP) and Django (Python) is (I think) the most familiar and widely used. By default will tackle most of your needs and be familiar enough to have >90% problems you faced tackled by others (and solved by googling). Make sure you insist to use Relational Database to save headaches along the way (PostgreSQL/Postgre and MySQL is the two most common ones).
As you are going to a dev shop, make sure that they are sticking to at least the above constraints. By having those, you can shop around for other dev shops in case you need to expand/mismatch down the line.
For now, your focus as a founder is to get the main product off the ground, and performance is second to that. It's a "good problem to have" and hopefully you'll have enough resources with the product success to improve it. Generally you can throw money at scale as long as you are not as big as 100k MAU, by throwing in more server/bigger servers.
For the specific dev shop, troy's comment is great to pick one. But for the framework, I recommend leaning to the ones I mentioned.