def downloaded electrum.org
i'm new to this is posting my receiving address safe?
Yes, it's safe to share your address it is a public address where we can check on blockchain if there is transaction happen with this address.
What do you mean by USB dongle? Are you using the internet from that USB? Does it seem that you are talking about USB stick modem where you can insert a sim card?
Maybe your internet from a USB stick is slow I have an old USB stick with slow internet because it gives me 3g internet signal which is too slow compared to LTE smartphones.
Can you try to change your DNS to google DNS? Sometimes it's DNS issue so changing the DNS to Google DNS or Public DNS can solve the issue.
Here is the google DNS below.