Elfish scheduling

4*15, 9

4 coal to deliver, each take 3 elves 5 seconds, and
15 presents to deliver, each take 2 elves 3 seconds.
There are 9 elves in total, represented by the y-axis
while time is represented by the x-axis.
A "." is an elf on the sleigh;
A "-" is an elf delivering coal;
A "+" is an elf delivering a present.
The coal recipients are numbered 1-4
The present recipients are lettered A-O

1. Efficient scheduling:

(E)
1 .---------1---------2---------3---------4.
2 .---------1---------2---------3---------4.
3 .---------1---------2---------3---------4.
4 .+++++++A+++++++D+++++++G+++++++J+++++++M.
5 .+++++++A+++++++D+++++++G+++++++J+++++++M.
6 .+++++++B+++++++E+++++++H+++++++K+++++++N.
7 .+++++++B+++++++E+++++++H+++++++K+++++++N.
8 .+++++++C+++++++F+++++++I+++++++L+++++++O.
9 .+++++++C+++++++F+++++++I+++++++L+++++++O.
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 (T)
                                         20 seconds.

2. Naive, "coal-first", scheduling:

(E)
1 .---------1---------4+++++++D+++++++K+++++++O.
2 .---------1---------4+++++++D+++++++K+++++++O.
3 .---------1---------4......+++++++H+++++++L...
4 .---------2+++++++A+++++++E+++++++H+++++++L...
5 .---------2+++++++A+++++++E+++++++I+++++++M...
6 .---------2+++++++B+++++++F+++++++I+++++++M...
7 .---------3+++++++B+++++++F+++++++J+++++++N...
8 .---------3+++++++C+++++++G+++++++J+++++++N...
9 .---------3+++++++C+++++++G...................
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 (T)
                                             22 seconds