There is an issue where WoW Classic will freeze for short bursts (usually 30-60 seconds at a time). I suspect I can continue casting during this time period based on cooldown times I have seen after the game resumed but I have not seen anything concrete for this part.
The issue is isolated to Classic and not retail but it occurs if the WoW program files are on a hardrive other than the one with your operating system. My OS is on an SSD with WoW being on a separate HDD drive. Once I moved the WoW files to my SSD, the problem cleared up. Based on my reading of the below forum post it does not appear to be isolated to the drive type.
Others that have experienced this issue: https://eu.forums.blizzard.com/en/wow/t/wow-classic-freezes-sound-stutter-interface-animation-delay/81715/2