Custom preloader hungs forever on certain browsers

Hello, I recently added a custom preloader via WP Smart Preloader.
Apparently the whole thing works randomly. In particular sometimes the animation doesn’t end and the page hungs forever. Here is the code:

HTML:

<div id="preloader">
  <div id="loader"></div>
</div>

CSS:

#preloader {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: #203258;
}
#loader {
    display: block;
    position: relative;
    left: 50%;
    top: 50%;
    width: 150px;
    height: 150px;
    margin: -75px 0 0 -75px;
    border-radius: 50%;
    border: 3px solid transparent;
    border-top-color: #16a085;
    -webkit-animation: spin 2s linear infinite;
    animation: spin 2s linear infinite;
}
#loader:before {
    content: "";
    position: absolute;
    top: 5px;
    left: 5px;
    right: 5px;
    bottom: 5px;
    border-radius: 50%;
    border: 3px solid transparent;
    border-top-color: #e74c3c;
    -webkit-animation: spin 3s linear infinite;
    animation: spin 3s linear infinite;
}
#loader:after {
    content: "";
    position: absolute;
    top: 15px;
    left: 15px;
    right: 15px;
    bottom: 15px;
    border-radius: 50%;
    border: 3px solid transparent;
    border-top-color: #f9c922;
    -webkit-animation: spin 1.5s linear infinite;
    animation: spin 1.5s linear infinite;
}
@-webkit-keyframes spin {
    0%   {
        -webkit-transform: rotate(0deg);
        -ms-transform: rotate(0deg);
        transform: rotate(0deg);
    }
    100% {
        -webkit-transform: rotate(360deg);
        -ms-transform: rotate(360deg);
        transform: rotate(360deg);
    }
}
@keyframes spin {
    0%   {
        -webkit-transform: rotate(0deg);
        -ms-transform: rotate(0deg);
        transform: rotate(0deg);
    }
    100% {
        -webkit-transform: rotate(360deg);
        -ms-transform: rotate(360deg);
        transform: rotate(360deg);
    }
}

I suspect that I should add some js, but I can’t find the section on XStore..
Any ideas?

EDIT: some typos and I use a child theme