Hi,
I have try asking those questions, hopefully we can get answers here in real life examples, not claim.
My experience has just reach almost 5k records, without index, it takes around 15 seconds to filter data, I use paging, but after getting indexed it is around 7 seconds.
If your collection can be distributed to more collections then it will be one of the workarounds.
But like others have said, focus on getting the MVP running first, get more tractions as much as possible then worry about other things later.
If you are not building for MVP and already have certain customers, you might want to go ahead with more firmed solution.