Just realized that Due compilation was done with -Os -- I changed that to -O3 in "~/.arduino15/packages/arduino/hardware/sam/1.6.4/platform.txt":
$ diff platform.txt.orig platform.txt
22c22
< compiler.c.flags=-c -g -Os {compiler.warning_flags} -ffunction-sections -fdata-sections -nostdlib --param max-inline-insns-single=500 -Dprintf=iprintf -MMD
---
> compiler.c.flags=-c -g -O3 {compiler.warning_flags} -ffunction-sections -fdata-sections -nostdlib --param max-inline-insns-single=500 -Dprintf=iprintf -MMD
24c24
< compiler.c.elf.flags=-Os -Wl,--gc-sections
---
> compiler.c.elf.flags=-O3 -Wl,--gc-sections
27c27
< compiler.cpp.flags=-c -g -Os {compiler.warning_flags} -ffunction-sections -fdata-sections -nostdlib -fno-threadsafe-statics --param max-inline-insns-single=500 -fno-rtti -fno-exceptions -Dprintf=iprintf -MMD
---
> compiler.cpp.flags=-c -g -O3 {compiler.warning_flags} -ffunction-sections -fdata-sections -nostdlib -fno-threadsafe-statics --param max-inline-insns-single=500 -fno-rtti -fno-exceptions -Dprintf=iprintf -MMD
$
Sketch size increased from 11,624 bytes to 13,548 bytes, but is still only 2% of the 512MB available on Due.
Runtime decreased from 548μs to 494μs, so now 83 x TIntel_2.8GHz > TArduino_Due:
47| 29|101|
113| 59| 5|
17| 89| 71|
494us
Hermann.