Used to use LagFix (requires root) on my Galaxy Nexus to fix (eMMC) slowness issue, if u dun wish to root, then dun need to manual fstrim
Nexus Storage Performance Over Time & TRIM 20130829 - xda-developers
Even though Android 4.3 and later is supposed to include TRIM automatically, sometimes it just seems it isn't working and we might need to manual TRIM..
As for the Gallery app being slow, its due to toooo many pictures over time, and Gallery app is trying to generate a thumbnail for each of your pictures so you will experience "slowness" while it is in fact busy generating thumbnails for ALL your pictures..
I haven't used Gallery for a long time, as I use Quickpic, don't have this issueor quite little of this issue popup..
Thanks a lot for the informative post!