Thanks for sharing your experience! While working with Microsoft Support they informed me that the tablet part of the 15" does not include a fan anymore (only one model of the SB2 15" included a fan in tablet). I'm currently waiting on a replacement SB3 13" to try and see if I experience the same issues.
@Waaimasjien - what temperature is your room at when you experience the issue?
@All - Is there anyone with a 15" SB3 that isn't experiencing throttling under heavy use?