Owners of original Nexus 7's with sluggish performance should see a speed boost after the update
It seems that the curious case of the sluggish Nexus 7 has been solved with the inclusion of TRIM support in Android 4.3. TRIM is essentially a way for the device's software to communicate with the on-board storage to let it know when blocks are sitting unused and ready for garbage collection. This is a necessary process, because when a user "deletes" files on a device, the file isn't actually removed from storage but simply marked as being available space to be cleared later when it is needed by the system. Unless TRIM is working to actually clear up those unused blocks, they will still be considered full by the storage itself.
For previous versions of Android TRIM wasn't enabled, which led to slower and slower I/O performance over time -- something that hit the Nexus 7 particularly hard. Luckily thanks to some investigation and testing by AnandTech, it is confirmed that the Android 4.3 update is bringing TRIM support to not only the Nexus 7 but every Nexus device.
Once updated to the latest version of Android, TRIM will attempt to do this garbage collection roughly once every 24 hours depending on the circumstances. If the device has been inactive for at least an hour, hasn't checked for criteria to clean in the last 24 hours and the device is either charging above 30-percent battery or not charging above 80-percent battery, the system will go through the storage and clean up unused blocks.
Because this is an issue that is only noticeable when a device has been used without wiping for some time, it may not be immediately apparent that TRIM is doing its job after the update. What you should see after moving to 4.3 is more consistent performance over time on your devices no matter how much data you move around on the storage.