Sorry, took longer than anticipated.
Now, this makesd command bakes a bootable µSD as described:
DEV=/dev/sdb makesd -f ext3-j -s 95 -r https://slackware.openpandora.org/pandora/sl4p/sl4p-15.0/rootfs/SL4P-15.0-alpha2.tar.xz -f swap -s 5
It boots but is indeed very slow and needs approx 120s to...