Well this depend of you cell line that you are working, but the slow growing usually is related to a senescence status, in this case you can realize a SA-b-Gal stained for confirm that you cell is become older.
This is a simple test. Seed a sample of the cells and incubate the plate in a tissue-culture incubator for about 10 hours. Examine the plate under a phase contrast microscope to see whether any cells have attached to the plate. If you see attached cells just pour off the unattached cells and subculture the attached ones. The unattached cells are senescent and are removed by this technique. This applies generally to all tissue-culture operations unless you have an unusual cell type.