[Users] How to hide sender IP address and host name?

[Removed GDPR]

I have just found that sending through Yahoo's SMTP does not reveal the
IP address of the sending host.

But I also found something else: Clicking on a folder in a Yahoo Mail
account with about 3800 messages takes extremely long (15-20 minutes)
and while this is running nothing else can be done with the program. In
comparison clicking on a Gmail folder with a similar number of messages
takes 2 seconds.

What could be causing this?