That's an extremely odd statement. In the first ten pages:
-there's absolutely NO reason to think the series will develop into an Harem
Well, it says so in the tags.
-is purely introducing the characters
It didn't even introduce anything except for the fact that Snakelady is madly in love with MC-kun. What it introduced was what kind of fanservice I can expect which was enough for me to just stop reading.
-We are presented straight away to a Lamia and the "Rules" of their biology, setting the story apart of other in the genre
And that's the problem, something considered normal (or a nice touch at most) in every genre is suddenly something that sets it apart when it happens in harem.
-The world building starts in full by the page 19 or so.
Doesn't get me anything if I get annoyed before it happens.
If you did not know before, Gengar is not really objective when it comes to content that he/she does not particurlary enjoy. Which is totally fine, people who waste time on things they don't like are wasting precious time that never comes back.
Well, obviously. Objectivity doesn't really do anything in explaining why I hate something. I'm not saying you can't like it, if you get your laughs out of Snakelady cumming because MC-kun rubbed her tail, then go for it I guess. To me humor and fanservice like that is just anoying though.
Also, I checked and it was literally 10 pages.