It’s a bit hard to tell what you’re doing as the screenshot has been scaled down, but I assume you’re using a material or a render target. In that case what’s even more effective than sampling with an offset like that is to use the lower res mipmap if it’s available. Try that and you should incur no speed penalty, since if they exist they’re being created regardless.
For a LONG time I’ve wanted to be able to access whatever mips of the postprocess materials exist by the way, I wish Epic would open them up for us.