In the case you don't have a super mega awesome gaming computer, you're game is usually going to run very slow. Optifine will help re-mediate this by improving the way Minecraft uses graphics and by allowing finer tuning of the graphic's settings.
GTNH is a very unstable modpack when it comes to adding on outside content. Because of this, it's important to install the correct version of Optifine, so follow the instructions below for help.
1) Download OptiFine 1.7.10 HD U D6 (Link) - We are running Optifine for Minecraft 1.7.10, so we need to be sure to get the correct version. GTNH specifically prefers D6 of the 1.7.10 versions. This version is near the bottom of the page. See picture below.
2) Move mod to mod folder - I am only covering the default installation of technic launcher. You will need to navigate to %appdata% which is your roaming folder. From there, you can go to ~\.technic\modpacks\mcnewhorizons\mods\ and you will place optifine in this mods folder.
3) Restart Minecraft (If Opened) - If you installed OptiFine with GTNH open, you will need to restart the game.
4) You're Ready To Play!!