The new Nexus 7 is the hot tablet in the Android world right now and while there are some decent improvements in terms of the specs, that doesn't mean your older model Nexus 7 will instantly turn into trash. Then again, some of those older model Nexus 7 tablets have been suffering from serious lag issues in recent months.
If you happen to have a 2012 model Nexus 7 that is sluggish -- the Android 4.3 update should bring some improvement. It seems the issue of sluggishness with the Nexus 7 is due to a feature called TRIM. This is described as being something that basically acts as a garbage collector picking up the trash.
The folks at AnandTech mention how TRIM will run when your tablet is idle with the screen off and sitting with a battery life of 70 percent or greater. The issue with the previous model Nexus 7 is that while TRIM support has been around since Android 4.2 -- it wasn't enabled. And regardless of how good a feature can be, if it is turned off then it will not do anything.
Anyway, looking forward you will likely want to grab that Android 4.3 update because it has enabled fstrim. The one important factor to remember -- this is not an instant fix. In other words, you are not going to have a magically faster Nexus 7 immediately after your tablet first boots into Android 4.3.
Instead you should update, make sure you have a good battery life and let your tablet sit so this can work some magic. That all being said, while it is nice to see Google has addressed this issue with the 4.3 update, this was something that could have been taken care of in the past. There are some 'fstrim' related apps in Google Play, however those were limited to those who had rooted their tablet.