css-bg-fps

body{
  position: relative;
}
body::before {
    background: url("filename") no-repeat center center;
    background-size: cover;
    content: ' ';
    height: 100%;
    left: 0;
    position: fixed;
    top: 0;
    width: 100%;
    will-change: transform;
    z-index: -1;
}