A while ago I had a huge argument with a development team regarding the usage of https. Their major concern was that the impact on performance would be so big that their servers wouldn’t be able to handle the load.
Their approach was to use https just for the login sequence and plain text communication for everything else. And it was not like they didn’t understand the underplaying problem of sending session cookies over an unencrypted channel, it was just that they thought https is too much for the servers to deal with.
Doing some research back then, I found a paper from the 90s stating that the performance impact was between 10 and 20%. And that only because of the hardware (mainly) CPU available at that time. With the advancement in computational power that should have decreased over time.
And indeed, as of 2010, Gmail switched to using HTTPS for everything by default. Their calculation shows that SSL/TLS accounts for less than 1% of the CPU load, less than 10KB of memory per connection and less than 2% of network overhead. Of course there were some tweaks, but no rocket science involved.
1%, 2%, 10KB. Nothing. I remember somebody saying that 640KB ought to be enough for anyone :) Maybe he knew something. As you can see in the link, Bill Gates didn’t actually say that.
5 more years have passed since then, hardware is more capable, cheaper, so there’s no excuse not to use https.
I’ve seen poor implementations where all http traffic was passed over a secure channel, but not the .js files. Needless to say, a MitM attack can easily modify the .js on the fly and run code in the victim’s browser.
As a closing note, use https for everything, don’t invoke the performance issues, there’s no reason in the current era not to do so.