Take care with Futures created in scala for-comprehension...

Create Futures outside the for-comprehension, in order for them to start immediately their operations. If you create your futures inside the for-comprehension, your execution will be sequential and not concurrent ! This is a mistake very easy to make, not so easy to detect because it won't fail, you just won't be able to use all your CPU as you should when the processing involves CPU, or for other cases check for example how many network connections are established simultaneously...