Well, according to some benchmarking I made with iozone, rootfs is definitely faster in reading than /opt.

Attached are the results of the following commands:

./iozone -f /opt/libQtWebKit.so.4 -a -+E -i 1 -i 2 -i 3 -i 5 -i 7 for opt.txt

./iozone -f /usr/lib/libQtWebKit.so.4 -a -+E -i 1 -i 2 -i 3 -i 5 -i 7 for rootfs.txt

the first column is data size in KB, the second is record len, the others are reading speeds in KB/s

I was not able to run iozone in sync mode with file in rootfs, it gives "invalid operation" error, so those results are not the real read transfer rates from the flash, but still are a good indication.