At the very least a group of drones with the necessary per-assimulation personal attributes, would have to be partially released/separated from the collective, in order to perform the required research.
When we first met Seven of Nine, it appeared that she had more autonomy that the bulk of the drones. So the drones don't all exist at the same level of mindlessness.
As you mention, we have 7 of 9 as an example. We also have seen several other examples of drones who are organized into subgroups with numerical designations. With this information and without further on-screen canon, it appears that drones are segregated into subgroups to accomplish specific tasks. While they would retain the memories and knowledge of the entire collective as a whole, the drones themselves would be focused on accomplishing their task. This would seem to me to be very efficient.
It's essentially multitasking.
Even Seven's full name (7 of 9, tertiary adjunct of Unimatrix 01) implies her role within the collective. She belongs to a group of 9 drones; the group serves as one of at least three other groups who performs various alternating functions (hence, adjunct) for the primary Unimatrix (as opposed to being permanently assigned to a specific task, like taking bullets in the chest or maintaining the transwarp or building interplexing beacons or whatever). At the time Voyager meets Seven, her temporary assignment is to assist Voyager with the 8472 problem.
At the exact same time, even though she's doing that one job, Seven has access to all the thoughts and memories and knowledge of the entire collective at her disposal. If someone working on some other task needed her input, they'd have it, even though she's currently occupied with the 8472 problem.
We also know that Seven hasn't ALWAYS been 7 of 9, that throughout her tenure as a Borg she's served in various capacities. In Survival Instinct, she's in a group of 5.
But, as the Queen so adeptly puts it in First Contact, it can't be explained in puny human terms, so it's not really worth the effort. They are many, and they are one.