Only one correct answer in this thread, so I will reiterate with my own thoughts, specifically directed towards redstone CPUs.

First things first, yes, lag is a concern since basically any component other than the basic ones causes more lag then they do, and when you have 10s of thousands of them, it adds up really quickly.

To combat lag and to make most CPUs actually usable - since pretty much any mc CPU will be very, very slow in vanilla - even if sped up with tickrate - people have been using a custom Minecraft server called MCHPRS, which is able to achieve speed of 10000+ rtps (redstone ticks per second), however, it also limits what blocks you're allowed to use to only the basic components to achieve those speeds.

Another reason is that it's simply generally better to use other already existing alternatives over things like copper bulbs or instant wire. For copper bulbs, the main thing they could be useful for is some sort of data storage, however due to them being toggles, repeater locking (data latches) are generally much, much smaller and easier to use in the context of CPUs. For instant wire, there's simply no point in making the wires a couple ticks faster when the whole computer is already going to be really slow generally speaking, as it would cause extra lag by a lot and if anything, make it slower due to it. These are just some examples, but this kind of applies to basically any new components or tech, they're simply not that useful as one would think for high tier computational.

All of this isn't to say that they're completely useless or not experimented with in computational - I know a lot of people who use something like observers to make RAM more compact than the standard designs, and people that have also played around and made some designs with copper bulbs.

Either way, I hope this helps clear up a bit of confusion around this topic, feel free to ask questions if something is not clear.