Is searching faster if indexed values are the same? Currently I have a collection events with an attribute createdAt of type date (for example 2019/09/09 10:32:12.123).
Because my users search a lot by date, I added an index on this attribute. There are probably hundred of thousands items in my collection and most of these events have a different createdAt date (which means a different index value).

My users are complaining that it takes too much time to retrieve some data, so I'm looking for a way to improve that.

As my users only search by date (never with time), I was wondering if creating another attribute 'createdAtDateOnly' (keep only the date and set the time to 00:00, for example: 2019/09/09 00:00:00) would be faster to search because many 'events' in my collection would have the same index.
Indexes perform better when they have unique values like customer ID, acc number etc. The more unique your indexed field is the less number of records gets picked improving the performance. You can build index on the date field, but would that be unique enough to filter the record you need?