FWIW, I’m seeing the same thing as johmue on at least two computers. Both are running Fedora/CCRMA, which comes with JACK2 (F20:184.108.40.206, F21:1.9.10), and both have i7 CPUs – an M620 from about 2010, and a recent 4790 – with 2 and 4 cores (with HT), and 8 and 16 MiB of RAM, respectively. The F20 machine is running a stock kernel; the F21 machine is running an “RT” kernel.
I did an experiment in which I opened the same project on both computers using the same version of Ardour (3.5.403) and exported the same 1020-second range from a stereo bus (not “master” in this case) to a 16-bit FLAC file, with a mix from 3 recorded tracks. There was one plugin in use - LinuxDSP’s MBC2B, pre-fader on the bus being exported. I did 3 exports on each machine: with the plugin enabled; with it bypassed via its own master bypass button; and with it bypassed using Ardour’s plugin bypass (the little green LED). Here’s a quick summary, showing elapsed time (manual stopwatch) for the export and a calculated speed-up relative to real time:
computer1: 141 s ( 7.2x)
computer2: 101 s (10.1x)
plugin bypassed (by plugin)
computer1: 126 s ( 8.1x)
computer2: 93 s (11.0x)
plugin bypassed (by Ardour)
computer1: 98 s (10.4x)
computer2: 69 s (14.8x)
In all these cases, computer2 looks to be about 1.4 times the speed of computer1 – I expected that ratio to be larger than that. (It’s probably coincidence, but that isn’t far from the ratio of their clock speeds (4000/2667), which I wouldn’t expect to be relevant.)
During each of the exports, the aggregate CPU load (according to top) hovered around 30-35% on computer1, and 20-25% on computer2, with occasional +/- 5-point excursions in each case. (I didn’t try very hard to eliminate other loads.) I also had gkrellm running, and from what I could see on its graphs, the load appeared to be fairly evenly distributed across all cores/threads. It never came anywhere near maxing out – on any core, let alone in total.
On computer1 (JACK 220.127.116.11, M-Audio FastTrack Pro - USB), qjackctl and Ardour showed 100% DSP load (in red) while exporting. On computer2 (JACK 1.9.10, M-Audio Delta 66 - PCI), they didn’t change much from their idle values – ie, about 7 - 9%.
I can try running with a locally-built JACK1 when I get some time here, to see if that makes a difference.