Question: I picked up the 2018 15" MacBook Pro recently, and right away I'm noticing the slightest amount of input delay on the keyboard. It doesn't always happen, but it's noticeable when it does - the typing cursor sometimes visibly lags behind when I'm typing.

It seems like it's happening a lot in Chrome specifically, though it happens in other applications. I could just be using Chrome a lot more than other applications.

Sometimes I can't tell if it's even input delay or just general slowness, for example when switching windows with CMD+TAB sometimes the window change animation is slower (it looks like I can briefly see the current window go from opaque to semi-transparent).

Has anybody run into the same problem, or does anybody have any ideas on how to verify whether this is happening? I notice it very clearly when I'm using my Mac, but since it's so unpredictable I'm skeptical of how reliable my observations actually are.

Answer: 
Yes, the keyboard lag exists, you're not imagining things … nor are you the first to notice this behaviour. This fluctuating-delay happens because there's a T2 chip in the way, and the keyboard and trackpad are processed by the T2's own BridgeOS first before reaching the rest of the system that macOS can see and have influence over. Oh, and before you ask, using a Bluetooth keyboard is not going to be faster.

Sadly, until Apple actually acknowledge this additional annoyance caused by the T2, nothing will be done about it.
Try using Safari instead of Chrome for a bit. Literally, ensure that it is NOT in the Dock or Activity Monitor at all. I'm betting the problem goes away. (Yes, I'm serious unfortunately). Chrome is an intense memory hog and has internal memory garbage collection that tries to run when the machine is idle. If you happen to begin typing or interacting just as it begins garbage collection, everything will lag for a very short time. If your lag is greater than 3 seconds though, you have other problems. 

You'll find many complaints that battery life is horrible when running Chrome, but is magically restored when running Safari. The two problems are intertwined.