Check that your download speed is greater than the recommendations (1.5 Mbps - SD, 4 Mbps - HQ, 8 Mbps - HD).

Test your internet download speed here: Internet Download Speed Test

These tests work by checking your "instant" download speed. However, your download speed WILL fluctuate over time.

If you are on wifi, switch to hard wire connection, or get very close to the wifi router.

Disconnect all other devices from the network (especially gaming systems and phones).

If none of the previous suggestions work, restart all of your network devices (modem, router, switch).

No. You can only be logged in on one device at a time. Other logins will be logged out automatically.