Yeah, quirky. Because Apple couldn't afford nor has the better software engineers than avast!; right? /s As far as I'm aware, avast! is writing software for an OS, not the other way around. This includes Windows, Linux, or any other UNIX-like or BSD derivatives. Just chiming in because of Apple's quirkiness. avast! decided that their web scanner should block iCloud Photo Library syncing. Disable the real-time scanners and, wouldn't you know, the iCloud Photos Library began syncing and had retrieved approximately 1k photos in about 1-2 minutes. I've been having this problem since 10.13.3. I'm going to completely remove the software and attempt another clean install. However, if it doesn't resolve then I'll try other antivirus software. I'll report back if the problem is non-existent.