I just updated Photoshop CC to version 21.0.0 on a Mac OS v10.14.6. I am constantly getting a dialog box with the message:
"Could not complete the Save for Web (Legacy) command because the document is currently being saved in the background. Please try the operation again after the save has been completed."
Of course, I understand why this message is showing. What I don't understand is why the app update would cause it to save files slower than before the update. Updates are supposed to improve the experience, right? Well, this is the opposite. Adobe - please fix the issue (and all issues) - preferably before releasing it as a consumer upgrade.