Having replaced the Intel SSD by a 256Go Samsung SM841N mSATA SSD, I can report that the issue lies with the Intel drive itself.
Slow boots cannot be reproduced neither with the Samsung mSATA SSD, nor with the Samsung PM830 SSD whether fitted to my SATA-II Latitude D630 or my SATA-III Latitude E6220. However, slow boots are reproduced with the Intel SSD when fitted to the D630 and the E6220.