why do you say it is delayed ?
sometimes, it’s just that the all the HO conditions are not checked (target cell congested or high-load, MS in umbrella and speed=fast, etc.). Therefore the BSC does not detect the HO, even though the radio conditions would be better in a target cell.
Try to perform the HO test at night, when there is no traffic load, in order to have a “simple” behavior of the network. Check also the QoS in the target cell, maybe it is too loaded to accept incoming HO.
Do you see any HO preparation failures (incoming/outgoing) in those cells ?