Improving performance
This mod is what shows the information on the top left corner of the screen.
It is quite handy, unfortunately it also causes a significant drop in framerates when open. This is not a fault of the mod itself and more of an issue with the vanilla game. There is a solution to have better framerates and the information always at hand.
Go into the game Options > Controls and find the InGame Info XML category, then bind the Toggle Overlay key to whatever you find most comfortable.
By pressing the button you can toggle the overlay on/off as you wish.
This is another handy mod, it gives you a minimap in the top right corner of the screen.
It also causes some loss of framerates, fortunately it's simple to get those frames back while also keeping the minimap functionality at hand.
You can switch between having the minimap enabled or disabled by using the appropriate key. This can be edited by going into the game's Options > Controls and under the JourneyMap category looking for Show / Hide MiniMap, by default it's CTRL + J.
The development team strongly recommends to NOT use Optifine. The mod is known to have severe compatibility issues which unfortunately cannot be solved. Adding Optifine will likely result in bugs and problems that we most likely will not be able to help you with. If some issues have known fixes they will be added to this page.
Broken skybox:
This is caused by an incompatibility between Advanced Rocketry and Optifine shaders. You can solve it by going into config\advRocketry\advancedRocketry.cfg and setting PlanetSkyOverride and StationSkyOverride to false. Keep in mind that this will force the game to use the vanilla skybox even in space.
Broken GT machines:
GT machines have special rendering with built-in shaders which can have issues with Optifine. You will have to disable them, to do so go into Mod Options > GregTech > Config > client options > shader options and set useShaders to false.