Welcome to the forum.
Who is your house builder?
We like to name and shame them as a warning to others. It also helps establish patterns and history useful for those with similar problems.
I have not heard of any rushing of homes to complete therefore avoiding the 3% additional Stamp Duty hike for second homes. It would not be a cost to the builder but I can see some buy to let "investors" stipulating completion to save them the extra cost.
The knock-on effect would be more likely to mean delays to owner/occupiers being completed, than their homes also being rushed, as house builder's resources would be prioritised for the buy to let plots.
That said it does give the house builder a convenient explanation/excuse for overlooking quality and remedying snagging in your home.
All new homes should be virtually defect-free but 93% have several defects all because the house builder and their site staff do not care enough to check the homes they build before their customers move in.
Post-occupation remediation of snagging items is an indication of failure of the part of the house builder.
Compounding this by refusing to carry out these works just adds to their overall failure and indifference to the people that buy their homes.
My advice to you is make a fuss. Keep everything in writing, and write to the CEO at their Head Office if the local staff are giving you the run around.