Splitting User Stories

If your team is having trouble finishing the work, I don't understand why your first thought is to split the work horizontally. A horizontal slice of work is usually not valuable to stakeholders, so I'm not sure why you would want to do that instead of understanding why the work can't be finished and finding ways to have thin, vertical slices of work that you can get done.

And these stories dont go into production until the quarter ends

Why? Does it take 3 months for the Developers to finish any piece of work to a usable standard, or is there some other reason for this quarterly constraint?

I would like to question the "extensive unit testing".

Unit testing is key to me; it is the safety net for Developers. As soon as something is broken, the unit tests should show this.

Why though do these take so long? It feels that there is a problem with the responsiveness of the unit tests. I assume these are automated. If not that would be a good start. If already automated, but too slow, investigate what takes them so long and optimize them. The book Software Engineering at Google give some good guidance for unit testing.

Hope this helps.
Grtz
Steven

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How does "splitting" the "user stories" solve a time problem? It creates MORE work.

How does the team work, is an item assigned (when it's picked up) by one developer, or do several developers work on 1 item?