Safari slow to load, flooding system.log with launchd errors Safari has become very slow to load pages. 5 to 10 seconds to begin to display a page. The problem is isolated to Safari on this computer. Other systems and browser are ok.

The system is a 13" Early 2011 MacBook Pro running El Capitan 10.11.6. I have done all of the stuff I would normally do to diagnose as problem: run Disk Utilities, purge caches, check permissions, login via a clean user account, etc. 

In Console.app I notice that the system.log is getting filled constant stream of with messages from launchd related to Safari. Most of these are related to WebContent and WebFeedParser and History. 

9/22/16 6:30:28.428 PM com.apple.xpc.launchd[1]: (com.apple.xpc.launchd.
domain.pid.WebContent.2063) Path not allowed in target domain: type = pid, 
path = /System/Library/StagedFrameworks/Safari/SafariShared.framework/
Versions/A/XPCServices/com.apple.Safari.WebFeedParser.xpc/Contents/
MacOS/com.apple.Safari.WebFeedParser error = 147: The specified service 
did not ship in the requestor's bundle, origin = /System/Library/
StagedFrameworks/Safari/WebKit.framework/Versions/A/XPCServices/
com.apple.WebKit.WebContent.xpc


To a lesser extent there are also ones related to ImageDecoder:

9/22/16 6:03:05.314 PM com.apple.xpc.launchd[1]: (com.apple.xpc.launchd.
domain.pid.ImageDecoder.1379) Path not allowed in target domain: type = pid,
path = /System/Library/StagedFrameworks/Safari/SafariShared.framework/
Versions/A/XPCServices/com.apple.Safari.History.xpc/Contents/MacOS/com.
apple.Safari.History error = 147: The specified service did not ship in the 
requestor's bundle, origin = /System/Library/StagedFrameworks/Safari/
SafariShared.framework/Versions/A/XPCServices/com.apple.Safari.ImageDecoder.xpc


Any ideas of why these are getting through into the system.log, if they are related to Safari's slowness, and how to fix.
I was experiencing the same symptoms: sluggish Safari, with identical errors in the console. I was also experiencing launchd using exorbitantly high cpu capacity.

After much gnashing of teeth, I discovered my problem was a bad spotlight index on an external drive, and after re-indexing the drive, the problem went away. Launchd behaves normally, Safari is responsive.

I do however get the same error in console, although I get it less frequently. It comes in batches of about 20 at once, every 12 hours or so. There seems to be no effect on performance.

Particulars:

'09 MBP 13" running 10.11.6 with the latest public beta security update.

Hope this helps!