[NiteWave]

the load time 2.991s is fast enough.

hence these are not getting cached because somewhat fewer requests for them. Additionally, SATA drive might just also be hurting fast retrieval of smaller images.

so you think disk i/o may be the cause.
you can do more tests to confirm this : put all the static files in the page in /dev/shm, and add rewrite rule to fetch these files directly from /dev/shm. so this will eliminate disk i/o slowness issue.