About 300MB. I just tried to replicate using random data and failed, will look more later, mine is power-law distributed and might make a difference.
Used these commands:
cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 32 | head -n 10000000 > text
cat /dev/urandom | od -DAn |tr -s ' ' '\n' |head -10000000 | sed -e 's/$/\/9.0/'> num
bc -l num > num2
(that's to create decimal numbers)
paste -d, text num2 > textfirst
paste -d, num2 text > numfirst
time LC_ALL=c sort -n numfirst -r > output
real 0m10.712s
user 0m21.959s
sys 0m1.644s
time LC_ALL=C sort --field-separator=',' -k2 -n -r textfirst > output
real 0m9.638s
user 0m21.940s
sys 0m1.293s