Simple partial view to simulate Slimsy on Umbraco V9 - a simple Slimsy simulator and the example to use it. This is used mainly when you migrate a V7/8 to a 9th version

example.cshtml

@await Html.PartialAsync("SlimsyIt", new ViewDataDictionary(ViewData) { 
{ "publishedContent", yourImageAsPublishedContentObject }, 
{ "width", 800 }, 
{ "height", 600 },
{ "class", "optionalClassString" } 
})

SlimsyIt.cshtml

@inherits Umbraco.Cms.Web.Common.Views.UmbracoViewPage
@using Umbraco.Cms.Core.Media
@using Umbraco.Cms.Core.Routing
@using System.Web;
@using System.Text
@inject IPublishedValueFallback PublishedValueFallback
@inject IPublishedUrlProvider PublishedUrlProvider
@inject IImageUrlGenerator ImageUrlGenerator

@{  //Slimsy simulator
    var publishedContent = @ViewData["publishedContent"] as IPublishedContent ?? null;
    var width = @ViewData["width"] as Int32? ?? null;
    var height = @ViewData["height"] as Int32? ?? null;
    var classItem = ViewData["class"] as string;
    var widthStep = 160;
    var lazyLoading = true;
    if (publishedContent == null || width == null || height == null) return;
    
    var heightRatio = (decimal)height.Value / width.Value;

    var imageOriginal = publishedContent?.GetCropUrl(ImageUrlGenerator, PublishedValueFallback, PublishedUrlProvider,
     width, height, quality: 80, furtherOptions:"&mode=crop&anchor=center", urlMode: UrlMode.Relative) ?? "";

    StringBuilder srcSetsBuilder = new StringBuilder();
    var w = widthStep;
    while (w <= width.Value)
    {
        var h = (int)Math.Round(w * heightRatio);
        var cropString = publishedContent.GetCropUrl(ImageUrlGenerator, PublishedValueFallback, PublishedUrlProvider, w,
            h, quality: 80, furtherOptions: "&mode=crop&anchor=center", urlMode: UrlMode.Relative);
        srcSetsBuilder.Append($"{cropString} {w}w,");
        w += widthStep;
    }
    //remove last comma
    var srcSets = srcSetsBuilder.ToString().Substring(0, srcSetsBuilder.Length - 1);
}
<img data-src="@HttpUtility.HtmlDecode(imageOriginal)" data-srcset="@srcSets" width="@width" height="@height" 
loading="@lazyLoading" alt="@publishedContent.Name" sizes="auto" class="lazyload@(classItem!=null?$" {classItem}":"") />