Why does HIV belong to a group of retroviruses called lentiviruses?
In Latin, lente means slow, so lentiviruses are retroviruses that are characterized by long periods of latency.

lentivirus A group of retroviruses that include human immunodeficiency virus, HIV-1. They cause disease after a long incubation period.
latency A state in which a virus infects a cell but does not replicate.

-Janeway's Immunobiolgy, 8th Edition, Kenneth M. Murphy
HIV-1 is characterized by long incubation periods. It can often take months before the virus is detectable on HIV tests.
It is also known that HIV reservoirs will exist in a patient, such that even if they have undetectable viral loads when on Antiretroviral Therapy, once they come off of the treatment, they will begin to produce virons again. The likely reason for that is that the immune cells that they infect can be very long lived, and as the viral genome has been incorporated into the host cells genomic DNA, it is incredibly stable. It will stay dormant for years until it is able to replicate again.