Busybox grep in the dependency gathering routine will read between 1 and 2 packages per second. GNU grep will give you about 10 or 15 dependencies a second, almost 10 times faster. I believe GNU grep caches the file it is reading in RAM so it does not need to pull it from the disk on subsequent reads. Busybox grep will be used unless grep is imported which then by default GNU grep will be called on. The md5sum functions in deb2sce and debGetSize also show the same performance increase with an imported grep, so it is quite a time difference overall.
Below is an actual comparison of the two greps, each ran twice to eliminate cache factor.
root@box:/home/tc# time /bb/grep "Package: file$" /etc/sysconfig/tcedir/debian_jessie_main_i386_Packages
Package: file
real 0m 0.38s
user 0m 0.37s
sys 0m 0.00s
root@box:/home/tc# time /bb/grep "Package: file$" /etc/sysconfig/tcedir/debian_jessie_main_i386_Packages
Package: file
real 0m 0.38s
user 0m 0.37s
sys 0m 0.00s
root@box:/home/tc# time grep "Package: file$" /etc/sysconfig/tcedir/debian_jessie_main_i386_Packages
Package: file
real 0m 0.01s
user 0m 0.00s
sys 0m 0.00s
root@box:/home/tc# time grep "Package: file$" /etc/sysconfig/tcedir/debian_jessie_main_i386_Packages
Package: file
real 0m 0.01s
user 0m 0.00s
sys 0m 0.00s