Why hard? Client needs to fetch all metadata needed for sorting for every post created during entire lemmy’s existance on every discovered lemmy instances, which depending on algo you are using, might include comments metadata. To aid client-side sorting you would need server-side filtering, which will limit data avaliable to sorting algo. For example client-side trending algo would not show old trending post because it was filtered out.
So client-side sorting is basically running stripped version of instance without file hosting.