Beets are considered fairly quick growing compared to brassicas like cabbage and cauliflower, squash or sweetcorn. However, if you aren’t seeing seedlings developing into more mature plants and swollen roots within eight-ten weeks, there may be a problem with the site or the soil. You should test the ph of soil to find out if it is acidic, alkaline or neutral. Keep the soil at 6.5-7.5 to ensure you are growing better beets. If your soil is too acidic (below 5), you can add some lime. If your soil is too alkaline (above 8), you can add sulfur chips.
You might also be dealing with beetroots that are struggling with ground temperatures. Hopefully if you aren’t planting too early or if you have prewarmed the planting site, this shouldn’t be a problem. But beets will struggle if soil temperatures are under 45˚F. Conversely, they can struggle if temperatures exceed 80˚F. Irregular or insufficient watering can also slow down growth, so make sure your roots get a steady supply of fluids.