As you can check here https://hyperaud.io/lab/pbr-test/ with any browser except Safari you can modify plabackRate without video playing interruptions.

In Safari seems every time you modify playbackrate video is rebuferring (making a 0.5 seconds pause)

I would change video.playbackRate every 50ms to show a ramped slow motion or progressive slow motion of the video.

This javascript code does the job, but, unfortunately is not working in Safari (iOS or MacOS)

var rint=null;
function iniramp(){
//pt is the central time point where make the ramped slow motion
    var sr=pt-1.5,er=pt+1.5; 
    if(sr<0)sr=0;
//et is end time of video
    if(er>et)er=et;
    video.currentTime=sr;
    video.playbackRate=0.98;
    rint=setInterval(function(){doramp(sr,er)},50);
    if (video.paused) video.play();
}


function doramp(sr,er){
    var p,step=0.02,pbr=video.playbackRate,minrate=0.25,ct=parseFloat(video.currentTime);
    if (pbr<0.5)step=0.01;
    if (ct<pt){
        p=pbr-step;
        if(p<minrate)p=minrate;
    } else {
        p=pbr+step;
        if(p>1)p=1;
        if (ct>er){endramp();iniramp();return false;}
    }
    video.playbackRate=p;
}

Any ideas for Safari?