Below we have listed a few hints/tweaks regarding issues we have seen or just options you have.
Your free to close this at any time

BetterFPS Mod:
We added this mod to help increase the player experience. However, since no machine is the same, if you experience any issues regarding FPS
we advise that you first try the following:

1: Navigate to the mods folder where the modpack files are installed
2: Find the BetterFps-1.4.8.jar and double click the file
3: You will see a small window appear, on the bottom left, click "Test Algorithms"
4: Wait for the process to complete (timing will vary), this will test your hardware and give you the best Algorithm to use. 
5: You can select "Change Algorithm" there but to be safe check the mods config in-game that it matches
6: You must restart your game for the change to take effect

For all other rendering/fps issues:

-Always try to reduce any uneeded or unwanted video settings to see if it helps, some often settings that can reduce rendering/fps:
> UseVBOS (if you have a beffy GPU, your fine to leave on, if not, turn off. This settings reduces the workload on your CPU & RAM and reassigning it to your gpu memory, this is only recommended if you have an okay to decent graphics card)
> Turn down your render distance
> Set graphics from Fancy to Fast
> Turn off smooth lighting and clouds
> Set particles to minimal
> Disable mipmaps

- Non in-game things to try
> Disable anti-aliasing and anisotropic filtering in your GPU driver settings
> Do not run other CPU- or GPU-intensive programs while the game is open
> On Windows 10, disable graphical effects such as Windows Aero and taskbar transparency
> On Windows, open Task Manager and go to the processes list, find javaw.exe (the one the game uses), 
right click it and set its priority to high (not realtime).

~~ Most importantly, use the bug report feature in your menu ~~