Hi CJ,
One of the test surveys used by the Survey123 team is on the same order you describe and performs adequately with standard data storage. As you might expect, rendering of the Analyze and Data pages is a bit slower, but still adequate. One thing you didn't mention is the communications environment - will the surveys be sent as filled in over cellular, or will they need to be stored on device then sent in bulk? If the latter, I would be curios to know what the comms situation to the internet is at the sending location - if all 70 are sending simultaneously and the connection is relatively thin/congested, I could foresee some degradation.
That being said, I'm a little concerned on the basic logistics you present: 400,000 inspections to be done by 70 inspectors over a 21 day period (assuming working weekends). This is ~5700 inspections per inspector for the period, or ~270 inspections per inspector per day. Assuming a 12 hour work shift, that is about 2:36 (2 minutes 36 seconds) per form, not allowing for transport or breaks. Even with a simple, 5 question survey, this seems to me somewhat aggressive considering if the assessment needs to take place at the damage site (if we're talking about inspection from aerial photography, this becomes a bit more reasonable).
