It seems the new app is very broken when if comes to download speeds and limiters. See other threads.
I have found, with limiters off, my 1G connection only gets about 15MB/s after spiking initally to about 95MB/s. This tends to mess up my entire network somehow, and until I close and reboot the PC, my whole gateway chugs down to about 15MB. Returns to normal once the app is closed and affected PC rebooted.
Further, according to many reddit users, enabling the limiter but putting a high limit on, like 1000000, opens up downloads to full speed once again. Blizz has some work to do on this laucher… it has been riddled with bugs for some time.