2018 MacBook Pro 4K monitor lag

I have a brand-new 15" 2018 MacBook Pro that I'm connecting to a 4K monitor (via a USB-C to HDMI adapter). There's some noticeable display lag (e.g. mouse lag) when using the display at scaled resolutions (anything other than the monitor's 4K native resolution). I've seen some other threads online where people have experience similar issues on High Sierra (which I'm using) with older machines, and the common claim has been "your graphics hardware just can't handle scaling a display of that resolution." That argument wouldn't seem to hold water with the newest MacBook Pros; at least, I would hope that Apple spec'ed these things to work with 4K monitors at all resolutions. Has anyone else had and been able to resolve a similar problem? I'm wondering if there might be some simple solution that I'm missing (e.g. my machine needs to use the discrete GPU and isn't).