After upgrading my Mac mini to macOS Mojave 10.14.4, every time I restart the computer the process "warmd" keep busy CPU and hard disk for 10-15 minutes, with great "spooling" noise and slowing down all the finder. What's happened? Its a well known bug of the upgrade?
It's supposed to "warm" the cache with your most frequently accessed applications and files.
If you have a "spinning rust" disk drive instead of an SSD, I can imagine that it may hurt more than help.
On my 2017 15-inch MacBook Pro with SSD for storage, I see high CPU utilization immediately after reboot for a few minutes, but between the 8 threads and the very fast storage, I don't notice a significant slowdown.
I think warmd pretends to load each of your 10 most frequently used applications and all the files you normally have open to "warm" its caches. Theoretically this will help your machine run faster when you actually try to use those applications/files.
With an old hard disk instead of an SSD, this just gives the system a lot of busy work to do and may not be such a wise idea. Also, while an SSD can perform several reads and writes at different places on the disk with very little delay between operations, a hard disk can only do 1 thing at a time, so with thousands of requests in the queue, boot time will suffer. Please file a RADAR with Apple. I can't currently find a way to disable warmd. If you want your machine to be responsive immediately after booting, this is a step in the wrong direction, and is a serious goof on Apple's part.
Have a read of the following;
It has instructions on how to disable warmd. I can not confirm they are still relevant at this stage as I myself am installing the latest OSX as we speak. I will provide more details after that install finishes.
launchctl kill -9 system/com.apple.warmd
launchctl disable system/com.apple.warmd