RE: Reduce connection establish time

endnode — Wed, 24 May 2017 13:45:01 GMT

Oh yes, this is kind of known thing. The only mitigations seems to be:

Use the shortest adv. interval possible (to increase chance that GAP Central will connect).
Use as high scanning duty cycle as possible (on GAP Central side to increase chance that advertising packet will be detected).
Avoid any LL/L2CAP/ATT otpional procedures like PDU or ATT_MTU extension, specification version exchange etc.
Do as small GATT Server as possible (minimum of GATT Primary Service, GAP Primary Service and your own single Proprietary Primary Service with as few Chars/Descriptors as possible).
Make GATT Server static (= avoid exposing Service Changed Characteristic) so GATT Client may cache it and skip GATT Service discovery procedure next time.
Use GATT Service Discovery caching on GATT Client side if possible.
Avoid Security Manager procedures such as pairing/bonding unless you really need them.

As you can see not many things are in your hands on high-level systems such as mobile and desktop platforms (iOS, Android, Windows, MacOS, Linux...) That's why this usually leads to designing simple GATT Profile with all security and fragmentation/flow/scalability management done on APP layer and also often sticking with default PDU and ATT_MTU sizes (because their gain on throughput is only visible for long connections and transfers - which seems not your problem if you care about connection times;). But maybe this is 2-3 years old approach and modern BT 4.2/5.0 devices will use new tricks how to overcome this problem (I'm curious to learn!). As you can see