Updated to include time to build 1:1 previews. Operating temps and CPU usage was comparable to previously observed results for the export operation.
I had some time today and ran a comparison of the rMB with some other computers performing an export with Lightroom 5.7. I used a test library containing 215 20mp raw files aggregating ~4.5 gb. The CPU figures are CPU temps and % utilization shown by iStat, averaged by means of the Mk1 eyeball. The CPU temps and %s were roughly equivalent for both the exports and the 1:1 preview builds.
TLDR: the rMB did quite well; for what it is, there is a large degree of awesomeness in this little machine.
rMB (2015) - Core M 1.2 gHz/8 gb
1:1 preview time: 30:04 minutes
Export time: 31.15 minutes
CPU: ~91*C at ~55%
note on exterior temps measured with IR thermometer (note that these are F, not C):
palm rest: 86*F speaker grill: 100*F bottom/base: 92-97*F - the rMB never got more than a tad over body temperature, in spite of running maxed out for almost half an hour.
rMBP 13.3" (late 2013) - i5 2.4 gHz/8 gb
1:1 preview time: 25:17 minutes
Export time: 25:15 minutes
CPU: ~78*C at ~58%
Dell XPS 13 (2013) - i7 2.0 gHz/8 gb
1:1 preview time: not tested
Export time: 25.15 minutes
CPU: not observed
Mac Mini (2012) - i7 (quad) 2.4 gHz/8 gb
1:1 preview time: 16:54 minutes
Export time: 16:55 minutes
CPU: not observed
BYO Z87 - Core i7 4770K @ 3.5 gHz / 32gb
1:1 preview time: 10:08 minutes
Export time: 8:20 minutes
CPU: not observed
