Dolphin takes about ~1.5 seconds to start on my 8-core machine with ssd, and another second to if you pick File->New Window to open a new copy of dolphin. It really disappointing result, compared to similar file mangers. I've tried disabling previews and playing with 'Sorting Mode' and other options, nothing helps to improve startup time.
Looks like I've got same issue on 2 machines with Fedora 26 x86_64 with Dolphin 17.04.1.
*** This bug has been marked as a duplicate of bug 348521 ***