There is considerable lag when you start to populate sub docks/grid stacks. Sometimes it takes more than 5 seconds to open. You click on one of those 2 types and it just hangs for a few. As it opens, the icons slowly load in. It is even longer after a computer restart.

Here is a small video showcasing this issue. Or download video attachment.
https://streamable.com/rlxunz

Looks to me like you are putting icons on sub-docks to items on non-local hard drives (LAN) ?

Anyway, sub-docks don't have their icons cached in memory, Grid Stacks do. Try using a Grid Stack instead (the first time it opens it will still take some time to retrieve all the icons from slow storage, but after that it will be much faster as the icons will be cached in memory from then on)

They are on another SSD. All my storage options are SSD. Any way for them to be cached for Sub docks as well in an update? I prefer that over Grid Stack.

If they are all in local storage there is no way you would be getting that lag under normal circumstances, especially not with that storage being ultra-fast SSD drives. It wouldn't happen even if you were using much slower HDDs.

The only time I have seen something like this happen is when users put shortcuts on the dock to non-local drives, which is not recommended because Windows takes a long time to access those drives over the LAN for the first time, transfer the image data required for displaying the icon, etc...

So, something else specific to your system must be causing this (could even be a failing SSD, check your Windows Event Viewer). I suggest making a backup of your settings first (Preferences -> Advanced tab -> Backup) and then removing items from that sub-dock one by one to see if one of those items is causing the lag.

You could also try using customized icons (i.e. individual PNG files) for each of the elements on that sub-dock, which will probably be faster than extracting those icons from the item file itself.

So, basically what I am saying is that the lag you are experiencing when opening the sub-dock is NOT normal and seems to be system specific - unfortunately this means you will need to use trial and error to try and figure out the cause.

Either way please let me know what you find out.

Using custom PNG's is what has worked for me. I had a tab on a shelf that had all the MS Apps. The icons were ugly and were slow to populate. When I replaced them with custom PNG's the lag went away.

You can search on Google for custom PNG's. For Firefox I would search for "firefox icon png". Click on images and you will get your custom Icons/PNG's. If I can't find a custom icon I use a free program called BeCyIconGrabber. It is available from:

https://becyicongrabber.en.uptodown.com/windows

It will extract all the icons in both PNG and ICO format from any folder. I just tested the download and it still works and my antivirus says it is clean. Save your custom PNG's in the largest size available in a folder for future use. They sometimes disappear from the internet.

I will caution that custom icons are addictive and soon you may find yourself wanting to change everything on your system.