Whenever there is any power transfer from a plug to a storage, or from a storage to a point, FPS go down significantly.
I'm running the Direwolf20 1.12 pack, with several added mods.
I tested this in a newly created Superflat creative world.

I placed down a plug next to a generator (tried different generators, from different mods).
This itself did not cause problems. Also when placing a point and feeding into something else (I used a enderio capacitor) worked fine. Adding the storage to the network was okay as well, as long as all the input from the plug went directly to the point.

However, when the plug feeds power to the storage (even with nothing else in the network), or the storage to the point, I had massive fps drops. I get roughly 25-30 fps without active flux networks. When the problem occurs, it drops to 10-15 fps

Are there any mods known to cause problems with flux networks, that I could try removing? Or is there anything else I can do to try and pinpoint the cause of this issue?

Edit:
I had some more time so I did some more testing with subsets of mods, but at this point I don't think a mod incompatibility is the cause.

For further testing the following mods and versions were used:
Forge 14.23.5.2821
Flux Networks 4.0.13-30
Actually Additions r146
EnderCore 0.5.45
EnderIO 5.0.40

Testing setup:

4 Generators (Either AA Coal Generator or EnderIO Stirling Generator with a double layer capacitor)
4 Hoppers on top to fuel them
Redstone line from a button to simultanously add coal
4 Flux Plugs, each connected to 1 generator
Flux storage (both basic and gargantuan have been tested)

Actually Additions and EnderIO have been tested separately, so only one of them was installed at a time.

On pressing the button and starting the process, fps dropped from ~240 to ~180.
As soon as the generators ran out of coal, the fps recovered.
As soon as the flux storage filled up, the fps recovered.

The mouse was not moved during testing, to rule out that a different view (sky vs ground) affected fps.

Edit 2:
I realized I should have used most current versions of mods, therefore I updated Actually Additions to r151, EnderIO to 5.1.52, EnderCore to 0.5.73 and Forge to 14.23.5.2847.

To save time I tested with both AA and EnderIO installed at the same time this time.

Results were however identical to the previous test.

System specs:
Windows 10 64bit Home
Intel i7 950
12GB RAM
AMD Radeon R9 390 8GB
Java 1.8.0_51

If there is anything else you need to know, please let me know!

You should start a fps sampler with LagGoggles to see if it's caused by storage's renderer.

I tried using LagGoggles, but that does not seem to be it as far as I can tell, as the results without the Generators running are equal to the results with them on, at about 150 ns/F for the storage

The results are also visible here: https://imgur.com/a/SoVrn7H
First two images are with Generators running, second set is with them off.

Oh I tested it out, it's not a bug, but an update packet for renderer every time when energy changed. It's optimized now