What protocol does Google use for Gmail? (not IMAP or POP)
You can access gmail either using the web interface, Google's Android client or using IMAP. As far as I can tell, the web interface and the Android app uses a completely different protocol than IMAP -- they are not just interfaces on top of it. The reason I'm sure of that is because the Android app can without problem open a folder with 1m mail in < 3 seconds. No plain IMAP client can do that.
So my question is what is known about this secret protocol? Where is the reference documentation for it? Has it been reverse engineered? Does Google sanction its use?
arnt's answer provides an excellent method to test gmail's raw imap speed:
$ openssl s_client -host imap.gmail.com -port 993 -crlf
...
* OK Gimap ready for requests from 12.34.56.78
$ a LOGIN ***@*** ***
a OK
$ c SELECT "[Gmail]/All mail" !!!!
* FLAGS (\Answered \Flagged \Draft \Deleted \Seen)
* OK [PERMANENTFLAGS (\Answered \Flagged \Draft \Deleted \Seen \*)] Flags permitted.
* OK [UIDVALIDITY 673376278] UIDs valid.
* 1142417 EXISTS
* 0 RECENT
* OK [UIDNEXT 1159771] Predicted next UID.
* OK [HIGHESTMODSEQ 8670601]
c OK [READ-WRITE] [Gmail]/All mail selected. (Success)
The command I've marked, c SELECT "[Gmail]/All mail" takes about 20 seconds to complete. Since that time is larger than it takes for the GMail app on my relatively underpowered Android phone to startup and load the All mail label which does it in less than 6 seconds even after I purged its caches. The web client is even faster.
Unless I'm missing something basic this proves "beyond reasonable doubt" that Google's GMail clients does not use IMAP since you never ever have to wait 20 seconds for any SELECT command to complete.
Answer
The Android app (at least the ones I've used) use IMAP. You can verify this by running Wireshark on the server.
As to why the Android app is so fast - what I know is that it uses the SEARCH command to select the most recent n messages. Desktop clients such as Thunderbird or Outlook are much more heavy-weight and download headers and metadata for every message in the folder, despite recommendations for them not to.
A smartphone does not have the resources to store and process millions of emails (although more modern ones might be getting there) so the SEARCH approach allowed quick mail access for handheld devices.
Anyhow, Wireshark can reveal a great deal about the behaviour of IMAP clients and servers. If you're really curious, give it a shot. You can't do this if the server is Gmail, but you can try it out on another server (e.g. hMailServer).