Sure, I think you can eliminate appendcols altogether in this search. Try something like this:
sourcetype="my_source_type" source="frt" (url="/F-" OR url="/P-" OR url="/C-*" OR url="/main") | eval Identifier=case(url == "/F-", "Family",url == "/P-", "Product",url == "/C-*", "Category", url == "/main", "Homepage") | timechart avg(download_time) by Identifier
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