A quick fix for the HD texturepack performance bug. https://mojang.atlassian.net/browse/MC-13206

Texture.java

    public void copyFrom(int par1, int par2, Texture par3Texture, boolean par4)
    {
        if (this.textureTarget != 32879)
        {
            ByteBuffer bytebuffer = par3Texture.getTextureData();
            this.textureData.position(0);
            bytebuffer.position(0);

            /* Forge: 
             * 
             * Optimize these loops a bit for non-rotated textures.
             * Should help the FPS loss on higher resolution texture packs.
             * If it becomes a major issue we may have to look into a more 
             * optimized animation system. 
             * 
             * https://mojang.atlassian.net/browse/MC-13206
             */
            //update - added by retep998
            boolean update = true;
            if (!par4)
            {
                int scanSize = par3Texture.getWidth() * 4;
                int w4 = width * 4;
                int targetY = (par2 * w4) + (par1 * 4);

                for (int y = 0; y < par3Texture.getHeight(); y++)
                {
                    textureData.position(targetY);
                    for (int x = 0; x < scanSize; x++)
                    {
                        textureData.put(bytebuffer.get());
                    }
                    targetY += w4;
                }
                //This whole if block - added by retep998
                if (this.textureCreated && this.height != 1 && this.textureDepth == 1)
                {
                    update = false;
                	bytebuffer.position(0);
                    GL11.glTexSubImage2D(this.textureTarget, 0, par1, par2, par3Texture.getWidth(),par3Texture.getHeight(), this.textureFormat, GL11.GL_UNSIGNED_BYTE, bytebuffer);
                }
            }

            for (int k = 0; par4 && k < par3Texture.getHeight(); ++k)
            {
                int l = par2 + k;
                int i1 = k * par3Texture.getWidth() * 4;
                int j1 = l * this.width * 4;

                if (par4)
                {
                    l = par1 + (par3Texture.getHeight() - k - 1); //BUGFIX: targetY -> targetX and -1
                }

                for (int k1 = 0; k1 < par3Texture.getWidth(); ++k1)
                {
                    int l1 = j1 + (k1 + par1) * 4;
                    int i2 = i1 + k1 * 4;

                    if (par4)
                    {
                        l1 = (par2 + k1) * this.width * 4 + l * 4; //BUGFIX: targetX -> targetY and parens
                    }

                    this.textureData.put(l1 + 0, bytebuffer.get(i2 + 0));
                    this.textureData.put(l1 + 1, bytebuffer.get(i2 + 1));
                    this.textureData.put(l1 + 2, bytebuffer.get(i2 + 2));
                    this.textureData.put(l1 + 3, bytebuffer.get(i2 + 3));
                }
            }

            this.textureData.position(this.width * this.height * 4);

            if (this.autoCreate)
            {
                this.createTexture();
            }
            else if (update)//The if (update) - added by retep998
            {
                this.textureCreated = false;
            }
        }
    }