Minecraft: 1.12.2
Forge: 14.23.2.2689
McJtyLib: 1.12-2.6.7-SNAPSHOT (2.6.7)
Rftools: 1.12-7.33

When I open the interface of a Storage Scanner or Storage Tablet my fps drops from 60 to 30-25 and stays at 30-25 while it is openned.
The issue is further increased when viewing large inventories like a Refined Storage system or a tablet with a tier 3 storage module. Waiting does not make the fps go up or stabilise.

This does not seem to happen with other guis from the mod.

I am having the same issue. Same RFTools and McJtyLib versions as you.
It's currently happening in Enigmatica 2: Expert v1.22. I wonder if it's some weird mod conflict, since it works fine in other modpacks.

That's normal. It has to scan all those inventories so essentially you get the overhead of asking the inventories from all connected blocks

But should it drop the fps even with a mere tier 3 storage module (with only 54 items in it)?

Can you show me exactly how that is setup? What blocks and how the scanner is configured etc

Sorry for responding so late but I wasn't being able to upload any pictures.

The setup is as so(the rs controller is next to the storage scanner):

This is the gui in the storage scanner:

You can see that the fps is very low (14 fps) in this picture.

This is my tier 3 storage tablet. I added more items since the last time I commented.

You can see that the fps with the storage tablet's gui openned is also low.

This is the storage tablet with the remote upgrade:

Notice that the fps is also very low.

And this is the usual fps I get (vsync is set to on outside of minecraft):

Hmm, this might have something to do with the interface though. I have not seen any slowdown using regular inventories

Testing with just RS and Rftools (with no companion mods) doesn't seem to cause any slowdown but it might be that the system simply doesn't have as much stress.