I assume tluyben2 means that if the db's index is bad the query has to do a much more expensive scan of the database. By adding correct indices this can be fixed.
Sorry for not being clear and thanks for explaining.
An index isn't free so you can impact the write performance, depending on the workload and usecases slower writes could have more of an impact than faster reads.
But even in a read heavy application it's possible that your databases query planner is "tricked" into using a specific index which can actually be slower than another (or even none) so you should always measure the impact that adding an index can have on your application.