Btw, if anyone who is running an AMD card has a problem like this, where they experience ridiculously low frame rates/fps...
Add optifine to your mods folder and after you start up, go into your menus and look for a link that says "animations".
going into that sub-menu brings up a bunch of things on and off. I believe 2 of them have names that include "texture", and the one that caused problems in our case, was in the left column.
sorry for lack of specifics, this was all done on my wife's pc about a week ago when troubleshooting all the mods 3, so I don't remember all the details, but this setting *also* had to be changed when we switched over to a ftb modpack, so my guess is that anyone who has an AMD video card and is getting suspiciously low frame-rates, might want to try that.
For reference and/or background, her computer is pretty *old*; A sabertooth 55i/p55 with an i7 870 and 16gb of ram.
We built it back in 2012, but we recently upgraded it with an AMD rx 580.
(...please nobody scream "omg old hardware!!" or "omg cpu bound"; just don't. lol).
With ATM 3 running, her system was usually getting somewhere between 6 and 18 fps.
*once* I saw it jump to 24 fps. but other than that it wasn't getting over 20fps.
we tried a bunch of "tweaks". Raising memory amount <<---just seemed to make the system more unstable; Changing java startup arguments. <<--didn't seem to help, wife ended up disabling them because everything seemed to perform *worse* with every tweak or change, to the point where it started rarely getting over 10 fps and she reset everything back to how things were before we started trying to "improve" things.
Eventually I stumbled across a post about something to do with textures or animations or something, and amd, and we tried optifine.
She started by disabling all the options under "animations", and *INSTANTLY* jumped up to over 100fps.
It was very easy to enable/disable the animations in optifine, it didn't require restarting the game or anything, so I had her go back and enable them one by one.
As soon as she re-enabled the one I mentioned earlier, her game dropped back down to less than 20 fps, and disabling it again, jumped her back up over 100fps again. I don't know what the highest she has gotten since then is, but i know it can hit at least 120 in atm 3 with that one setting disabled. she ended up leaving all the other animations enabled, and I don't recall any other things/setting being turned down actually even making any noticable difference on her system. like render distance and stuff; we'd turned it down trying to see if anything helped, and since it didn't, we put it back up.
As far as performance after the change made in optifine, it ran great after that. So much smoother and better, it was like using a completely different machine. except for some sort of issue between mincraft, obs, discord (and/or skype) and twitch; which kept resulting in all kinds of glitchiness when she would try to live stream the game. But yeah, other than that, it played great. no more waiting for world stuffs to load, no more frame-by-frame panning, etc.
Anyway, like I said, anybody running modded minecraft on an AMD card, should probably grab a copy of optifine, hit f3, and start seeing how disabling things in that "animation" section to see if any massive fps changes occur, because what little pieces of info i found scattered around in random posts gave me the impression that this issue can affect both old and new generations of hardware.
Anywho, just wanted to pass that on. Take care and have fun :)