If you only need to write and never read you don't need database.
> /dev/null will suffice.
And if you ever need to read anything even once database without indexes is slow
> /dev/null will suffice.
And if you ever need to read anything even once database without indexes is slow
"database = fast" is way worse heuristic to believe in for the people that need heuristics to move on with what they are doing.
This may be true in most cases, but not all. It just depends on your access pattern. If all you do are full table scans (possibly feeding to hash-joins), you won't benefit from indexes at all!
The whole point is that indexes do not auto-magically improve performance with no downsides. If they did, we could just index all columns and call it a day!
In most cases it means that you shouldn't be doing what youa re trying to do. Or best case, that maybe database is not the right tool for your job.