I loved finding videos that I completely forgot I have made. Here is one:

EEVblog #1247 - DDR Memory PCB Propagation Delay & Layout

When does PCB propagation delay matter in PCB layout? Dave goes down the rabbit hole from DIY TTL processor design to DDR memory design and layout. DDR memory termination. What is a timing budget? When is it important? How does signal integrity matter? When do you have to do serpentine PCB traces to match trace and differential pair lengths?