"Fast charging" is not one specific protocol anymore - it depends on the brand and the specific model of phone, as well as which specific charger you're using. For Samsung for example there is now regular charging at 15W, 'fast charging' at 25W, and 'super fast' which garuntees 25W and can go up to 45W. But I believe a random charger that provides like, 18W, will still say "fast charging" so long as it is getting over 15W, but obviously an 18W charger is still going to charge slower than a 25W charger.