Hey,

It might be that there is an account limit @ your gdrive. It might also be the client you are using to upload the back'up. Try running this:

wget -O speedtest-cli https://raw.githubusercontent.com/sivel/speedtest-cli/master/speedtest.py
chmod +x speedtest-cli
./speedtest-cli

to see if there is a network issue. Running ./speedtest-cli -h will also provide you with a few options like choosing the location of the test servers.

As for the number of visitors, the limit depends on the application they are trying to access and the physical resources the droplet has, such as RAM, CPU, Disk speed. For new accounts there is also a Bandwidth limit which is stated on the pricing page.