15" 2.3GHz, 16GB RAM, 512GB SSD, 750M
CandyBox2 is an ASCII RPG that was actually using 20% according to Chrome Task Manager. I mention it in the numbers below because it actually affected temperatures.
With iTunes, Tweetbot, Textual, Chrome (19 tabs, including CandyBox2 in background) and no monitor plugged in (using Iris Pro), it's at 47C. Without CandyBox2, it's at 45C.
With iTunes, Tweetbot, Textual, Chrome (19 tabs, including CandyBox2 in background) and second monitor plugged in, it's at 62C.
Yesterday I played a 4K/"Original quality" video on YouTube while Time Machine backed up to a network share, also with second monitor plugged in, playing the video in fullscreen on the rMBP -- the hottest I've seen it -- 98C. Yeah, I realize that's pretty hot. But I don't anticipate any 4K video editing on this machine and HTML5 4K playback probably isn't as efficient at decoding the video as professional editing software.
Overall it's extremely quiet at 2000rpm and 62C. The fans can certainly get loud if you push this machine, but it's definitely capable! If the 4K video stuttered at all, I didn't notice. It was extremely fluid. I'd like to try playing a 4K video file (compressed, mkv or something) in VLC or another player with hardware acceleration and see how hot it gets. I was certainly surprised to see 98C... but it's not hot enough to potentially damage the processor unless it sustains that temperature for long periods of time. Even at that point I think the CPU would just turn everything off except the fans and cool off.
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Just started stressing it a bit, doing a large file transfer over 802.11ac -- I hit 65MB/s pulling down a VM image from my NAS which is AWESOME, but my CPU temp hit 90C which is a bit high. Granted, this is all with a second monitor attached so the general temperature of the chassis might be higher because of the 750M. AND I had CandyBox2 which was increasing the temperature more than I actually anticipated. After I closed it, my temps for the transfer are around 85C. Still pretty hot for a wireless file transfer, but maybe 802.11ac just uses that much power...
Just downloading Bioshock Infinite from Steam at 7MB/s bumps my processor to 70C.