The D-Visa being issue is to travel to and apply for the residence permit.
Once applied, a temporary residence permit will be issued for the expected time needed until the final residence permit is issued.
Since D-Visas are country specific, this may not be true for all countries but for France and Germany it is.
I have found the Portuguese SEF site that explains some new regulations about the residence permit process.
Up to now, after the initial arrival with the issued D-Visa, you are required to make an appointment with the SEF localy.
This appointment can now be done beforehand when the date of arrival is known.
The Immigration and Borders Service, in situations where it is informed of the travel date, will provide, in the several favorable opinions concerning residence visas, the schedule date for SEF. The applicant is, therefore, waived from scheduling when at the national territory;
The appointments for the granting and renewing of the residence permit can now be made, at the applicant’s request, for any regional directorate/delegation, allowing therefore to anticipate deadlines;
How the SEF then deals with the process is not stated directly, but Article 51 of
Regulatory Decree 9/2018, 2018-09-11 - DRE suggests that permits will be issued as needed.
If the application takes 6-8 months to process, then a residence permit for that period will be issued replacing the D-Visa (including it's conditions)