it will get busier. the goal is for write to cause the small load every time action happens. this way there is never single heavy query that would cause big load at read time. the main issue with MongoDB I've seen is that writes tend to slow down once you go over 1000+ keys in the aggregated payload. this can easily happen if you have something like yearly bucket. on the other side Redis is not affected by it as increments are done individually, so writes stay flat.

about your update: dashboards are the easy side here. rendering one is a lookup for few points from pre-aggregated buckets. all you need is a key and a timeframe with granularity. unless you are trying to get something like 1 month worth of per-minute data, it will be quick.

if you want real isolation, you can give driver dedicated user with its own limits or point it to dedicated database. we started with Redis to keep it easy, then Postgres as I wanted to ensure persistance and ended up with MongoDB which handled the writes with ease.