-icount increases boot time by >10x

When I specify the -icount option, some guest operations such as booting a Linux kernel take more than 10 times longer than otherwise.  For example, the following will boot Aboriginal Linux to the login prompt about 6 seconds on my system (using TCG, not KVM):

wget http://landley.net/aboriginal/downloads/old/binaries/1.4.5/system-image-i686.tar.gz
gunzip <system-image-i686.tar.gz | tar xfv -
cd system-image-i686
sh run-emulator.sh

If I replace the last line with

QEMU_EXTRA="-icount shift=auto" sh run-emulator.sh

booting to the login prompt takes about 1 minute 20 seconds.

I have tried different values for "shift" other than the "auto" used above, but have not been able to find one that gives reasonable performance.  Specifying "sleep=off" also did not help.

During the slow boots, qemu appears to spend most of its time sleeping, not using the host CPU.

I see this with multiple versions of qemu, including current git sources (c181ddaa176856b3cd2dfd12bbcf25fa9c884a97), and on multiple host OSes, including Debian 9 on x86_64.