Just updated from 1.2.3 to 1.2.4 and I experienced a huge FPS drop (119 -> 12). I have one flux plug in my base sending power to another dimension; as soon as I broke the plug, FPS recovered. Replacing the plug again took my FPS down to 12.

1.10.2 on FTB Beyond.

Now that I think about it I had some major stability problems when I placed a plug in the deep dark. Do you have any plugs in dimensions that you're not commonly in or in modded dimensions for that matter?

Seeing the same FPS drop with, but with Flux Points.

Same issue, major FPS drop in 1.2.4.
I'm on MC 1.10.2 on FTP Sky Factory 3.0.10
Originallly, it contains FluxNetworks 1.2.3.
I've updated mod manually to 1.2.4 because of fixed priorities, and FPS lowered from ~60 to ~10
I had to rollback to FluxNetworks to 1.2.3.

Adding my two cents to this to follow it up; updated to FTB Beyond 1.9.0 (Minecraft 1.10.2, FluxNetworks 1.2.4) and my FPS went from 120+ to less than 20 if I was near any active Flux Point. Breaking the point instantly restored my FPS (as long as no others were also near me), and instantly killed it if I placed a new one.

Using a mod that added the /csampler command (I wish I remember the name), I was able to enable showing render updates/block updates; the FluxPoints show up bright red permanently, indicating they're firing block updates constantly, which explains why I was also seeing Chunk Update counts upwards of 20-30 in the F3 debug menu. I wasn't able to downgrade back to 1.8 to check if this was occurring previously, but we had debugged a similar FPS drop issue (traced to Botania) two weeks ago and I don't recall seeing the flux-points in red back then, or I would've removed them as we tried to fix the lag.

I was able to somewhat solve the massive FPS drop by moving the entire power setup (capacitors, generators, fuel, etc) to a sky island away from the base, but this was completely unnecessary in 1.2.3 so something's definitely changed between then and now.

Edit: I should note that my network didn't have any interdimensional stuff attached at the time either; it was just a way to move power from our reactor setup a few dozen blocks away to our "machine room" because making a wire that long was decidedly ugly.

Apologies, fixed in dev.