What is the point of this post? o.O
I can assure you, that you probably read many bad examples, there are also good ones.
And please remember that shounen romcoms also include those very (*cough*) high quality (*cough*) harem and ecchi series. I dare say I had to avoid more bad shounen romance than I read bad shoujo romance.
In addition both demographics utilize that aspect completely differently. In shounen you often have romance as a useful (better: convenient) tool to drive the plot forward or for comedic relief, while shoujos often focus heavily (or only) on the romance. That takes more time because character development is THE most important thing. It's finding, realizing and growing through love instead of keeping it (there are good examples for that too though).
Compare that with shounen where characters or personalities often stagnate, or they change because of external influences, not because they want to change.
And who said slow romances are realistic? From my experiences attraction is fast, but more stuff evolves pretty quickly from that. You can only call it slow if both people are so damn shy that they don't say anything for years... Why would that change for adults? I'd say that's even faster for them because they are more direct in expressing what they want because they personalities are fully developed.
btw: don't read 100+ chapter romance shoujos. no matter how good the character development is, there is a point where things can only get worse.
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in need of romance?
uncommon,
sad,
sweet