Time fill stations are used primarily by fleets and unlike fast fill stations, they may have larger compressors and the vehicles are generally filled directly from the compressor, not from fuel stored in tanks. Vehicles may take several minutes to many hours to fill, but typically get a fuller fill then with fast fill.
Fast fill stations receive fuel from a local utility line at a low pressure and then use a compressor on site to compress the gas to a high pressure. Once compressed, the CNG moves to a series of storage vessels so the fuel is ready to go for a quick fill-up. Fill times are typically less than 5 minutes for a 20 gallon equivalent tank.
Source: Mansfield Energy Corp.