If done SQ turnaround at MAN a couple of times. That is scheduled at 90 minutes. I only had 5 minutes or so in the lounge before it is time to board. So I would say 40 minutes is too tight. Unless the airline sends ahead info on connecting pax, the transit security is unmanned so there is a long wait for them to get the officers. The security check is quick and easy - it is getting the officer that takes the time. SQ is able to issue BP at the gate - I presume KL can also? There is a transfer desk right by the security check, however I dont think they can issue BPs - they seem to just check pax are eligible to transfer and call an officer for the security check if needed.