import math


def _c3857t4326(t1, t2, lon, lat):
    """
    Pure python 3857 -> 4326 transform. About 12x faster than pyproj.
    """
    xtile = lon / 111319.49079327358
    ytile = math.degrees(
        math.asin(math.tanh(lat / 20037508.342789244 * math.pi)))
    return(xtile, ytile)