the text version of 辭源 uses a css counter and it also happens to load slowly. while the counter should not incur a performance hit, I wonder whether it actually does.
in any case, my own practice is to replace counters with the actual number. I see no reason to use counters if the data is stable and the sequence never changes – as with dictionary entries. a dictionary is not like a regularly updated log that needs to be dynamically re-numbered all the time.
also, converting the html counter to actual numbers makes the raw data more useful, so you can extract the entry number as part of the dictionary info when needed.
hence I do not use counters for dictionaries, where it is just fancy html that serves little purpose.