Ways to improve makespan in MILP

Hi!

I'm trying to optimize the makespan of M tasks with time resource constraints (FTE availability). In which it is possible to make parallel tasks (if the FTE by hour allow it). There's no order or precedence constraints. So the output is the scheduling of the M tasks.

I was trying a time-indexed approach; using a tiny sample data set, it runs smooth but when I try M > 20 and T > 100, and it takes too much time to solve it (2 hours and running...)

It is posible another approach in the model to solve it?  

Any help will be welcome.

Thanks!