I have found no solution for flipping a texture in Unity that is particularly efficient without going into multi-threading, which may or may not be possible depending on the project at hand, as to truly flip the texture you have to move all the pixels in the image around to "flip" the image. I've attempted to create the most efficient method for…

TextureFlip.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class TextureFlip
{
    public static Texture2D FlipTexture(this Texture2D original)
    {
        int textureWidth = original.width;
        int textureHeight = original.height;

        Color[] colorArray = original.GetPixels();

        for (int j = 0; j < textureHeight; j++)
        {
            int rowStart = 0;
            int rowEnd = textureWidth - 1;

            while (rowStart < rowEnd)
            {
                Color hold = colorArray[(j * textureWidth) + (rowStart)];
                colorArray[(j * textureWidth) + (rowStart)] = colorArray[(j * textureWidth) + (rowEnd)];
                colorArray[(j * textureWidth) + (rowEnd)] = hold;
                rowStart++;
                rowEnd--;
            }
        }

        Texture2D finalFlippedTexture = new Texture2D(original.width, original.height);
        finalFlippedTexture.SetPixels(colorArray);
        finalFlippedTexture.Apply();

        return finalFlippedTexture;
    }
}