lsws don't cache static content since it's not needed at all or in most scenarios.
lsws serve static content quickly and consume little system resource.
when a static content i.e., an image is requested repeatedly, linux may cache the image in memory, so lsws serve the image more quickly. it's automatic, and won't consume lsws process's own memory. so if lsws serve static contents only, you will notice lsws process keep on low memory and low load all the time and yet very fast. or in other words, the static contents are cached by linux kernel automatically. lsws just make good use of the linux kernel file system's great efficiency. if you wish some static content always in cache(or memory), you can put these files under /dev/shm, no need ask lsws to put it in memory as well.