The case of the terribly slow PayPal emails

You shouldn't rely on paypal's email anyway.

Use their instant notification API https://paypal.com/ipn to have it trigger a script on your server to notify you any way you'd like.

You get more info from the api too and it's triggered before emails.

The article is about an investigation into why outlook was taking a long time to open paypal emails.

It is not related to relying on paypal's email for payment processing.

TL;DR: Paypal is embedding an image into its emails using the "//" shortcut, instead of specifying http: or https:.

This is wrong since it assumes the mail user agent loaded the content from a web server. There's zero benefit to dropping http: or https: in a URL in an email.

What are 2o7.net & omtrdc.net?

http://www.omniture.com/en/privacy/2o7

Everyone should block all access to 2o7.net, the same as Google Analytics. Nothing useful comes from them.