What you're asking for is a change in the way WACUP works, not a fix. Until now, you didn't make it clear that you wanted to transfer more than 1 item from a library view to the playlist editor at the same time. The "Play all items ..." option would transfer ALL the items in your library beginning with the item double-clicked on. Since your library contains many thousands of items and playback is currently designed to start after the transfer is complete, it will take a few seconds.
If your files are stored on a mechanical hard drive, moving them to a solid state drive would shorten the delay. In this case, it is the read/write speeds of the storage drive and not the power of the CPU that matters.
The transfer does not have to be 1 or all. You can select a group of items to double-click on. Be sure to disable the "Play all items ..." option. You select a group of items in a WACUP view the same way you would select a group of items in Windows Explorer. For a group of consecutive items, you select (point to or click on) the 1st item, then hold down the Shift key and select the last item in the group. For items that are not consecutive, you select the 1st, then hold down the Ctrl key while selecting the others.
Since the transfer of 1 item should take a few milliseconds, the transfer of a few hundred or less should take less than a second.