For my AS-202T, the larger the library the longer it took to make any page to appear. I reached a point where I was no longer getting reliable page updates, and that’s when I upgraded to the 7004T. The size of your metadata folders is going to be one of the key determining factors here. If you have thumbnails being generated, that 's adding a lot to the size of the folder. And increasing the odds that the machine is going to slow down so much it can no longer keep pace with your viewing changes. (Or drop the server completely.)
The AS-1002T has a somewhat better CPU than the 202T had(s), but the bottom line is simple. The machine is barely capable of running Plex with a moderately sized library. (Including photos and music in there, BTW.) Accessing the pages is your first indication you are over-taxing the system.
If we compare your NAS to a car race, you are running a Ford Pinto. I’m running a Firebird. And folks with higher end CPU’s are running Lamborginni. All three cars have the same basic parts. 4 wheels on the ground, an engine, steering wheel, etc… But as the price tag goes up the level of performance rises as well. I can lap you twice in the time it takes you to make one lap. And get lapped 2-4 times in the same period of time.
In other words, it comes back to the same thing I already mentioned to you. The CPU in the 1002T is just too lightweight for any medium sized library. (> than 1000 items, < than 2500 or so.) Use the device to store your media on and set up another machine as your PMS, pointing to the media on the NAS is your only workable solution with that device. Otherwise you are going to continue to have problems, and likely won’t be happy with the NAS or with Plex itself. (And the reality of the situation is, the hardware just hasn’t got the power it needs to do the job you’re asking it to do.)
And trust me, if there were a way to make this work on your NAS I would already be doing it on my 202T…