JBake: list all posts with a particular tag (tagged_posts)

In freemarker, how do I iterate over all blog posts with a particular tag, for example the tag "algorithms"?

<#assign relatedBlogs = ...>
<#if relatedBlogs?size &gt; 0>
    <h2>Related blog posts</h2>
    <ul>
        <#list relatedBlogs as blog>
            <li>
                <a href="${content.rootpath}${blog.uri}">${blog.title}</a>
            </li>
        </#list>
    </ul>
</#if>

This doesn't work for me:

<#assign relatedBlogs = tagged_posts["algorithms"]>

This doesn't work either:

<#assign relatedBlogs = tags["algorithms"].tagged_posts>

tags is a sequence, and to use tags[...] you need a hash. I ended up getting it to work through ?filter()and ?first:

<#assign relatedTags = tags?filter(tag -> tag.name == "algorithms")>
<#if relatedTags?size &gt; 0>
    <#assign relatedTag = relatedTags?first>
    <h2>Algorithms related blog posts</h2>
    <ul>
        <#list relatedTag.tagged_posts as blog>
            <li>
                <a href="${content.rootpath}${blog.uri}">${blog.title}</a>
            </li>
        </#list>
    </ul>
</#if>

This won't scale well, because filter() iterates the tags for every page, but it works fast enough for websites with hundreds of pages. Here's the related issue.