I use a lot of sub-docks on my main launcher. But when I first start my computer or after I haven't accessed the dock for a while, there is a significant lag (a few seconds) before the icons load in for a sub-dock.

I've put an example on YouTube here: https://youtube.com/shorts/NHMDr9FA7RM?feature=share

Is there a way to avoid this? I would rather it held onto a bit more memory, or whatever it is, in order to keep my bar snappy.

Nothing to do with memory.

My guess is that one (or more!) of the shortcuts on that sub-dock point to items stored either on the LAN (i.e. non-local drives) or on a HDD that stops spinning when not in use for a while.

The sub-dock needs to retrieve the icon for that item, and it can only do it by accessing the target file (unless you customize the icons for those shortcuts with stand alone PNG versions stored on fast storage, which might be another way to solve the problem).

If the target is located on, say, an external hard disk set to stop spinning after being idle for x amount of time, the dock has to wait for the disk to spin up before it can retrieve and display that item's icon.

This issue is already "minimized" by the use of those generic icons - if the dock notices it is taking a long time to retrieve icons, rather than making you wait with zero feedback for all the icons in that sub-dock to be retrieved, it shows the dock ASAP using those generic icons as placeholders, and then replaces them with the correct images one by one. But at least you get to see something happening rather than clicking to open the sub-dock and absolutely nothing apparently happening for a few seconds (or however long it takes for all the icon images to be retrieved).

Also, unlike shelves, sub-docks do not cache icons in memory. The reason the sub-dock opens faster the second time around is because Windows itself is caching whatever that sub-dock points to. After a while it might discard that information, and the sub-dock will again take as long to fully display the icons as it did the first time.

Anyway, you like to crop stuff so I can't see which icon/item in the sub-dock is the first culprit - but the (first?) problematic item is the last icon with an image just before the first generic icon.

If the items are in non-local hard drives (i.e. in the LAN) then that is a no-no for dock items precisely because of this. You can watch something similar happen if you open the Windows Network Neighborhood, it will be *slowly* populated as Windows discovers all the network items.

That makes perfect sense. Thanks for taking the time to explain it.

Lots of my utilities and things are on HDD, to try and keep my system's SSD relatively tidy. I'll probably create an icon folder on the SSD as you suggested.