For some time I've been experiencing really slow folder opening in Finder and Open/Save panels. Clicking to open a folder would take anywhere from a few seconds to a minute to appear. I lived with this for some time, until I was debugging something and went to watch my syslog.

As it turns out, this is caused by a corrupt CloudKit database. The cloudd daemon repeatedly crashes trying to read it and then gets re-launched by launchd. Here's an example of the syslog message:

Apr 06 23:05:14 yivo cloudd[426] <Error>: *** Terminating app due to uncaught exception 'NSGenericException', reason: 'Error executing SQL: "begin exclusive" (266) - errcode:010a, msg:"disk I/O error", path:/Users/USER/Library/Caches/*/CloudKitMetadata'

Trying to read the database with sqlite3 proved my suspicions further:

/Users/kirb/Library/Caches/CloudKit$ sqlite3 CloudKitMetadata
SQLite version 3.8.5 2014-08-15 22:37:57
Enter ".help" for usage hints.
sqlite> .dump
PRAGMA foreign_keys=OFF;
/**** ERROR: (10) disk I/O error *****/
ROLLBACK; -- due to errors

The fix - just delete the database. OS X will download and recreate the data for you.

In Finder, press Shift-Command-G. Type in: ~/Library/Caches/CloudKit. When it eventually loads, move CloudKitMetadata, CloudKitMetadata-shm, and CloudKitMetadata-wal to the Trash. Alternatively, open Terminal (it's in /Applications/Utilities), and enter:

rm ~/Library/Caches/CloudKit/CloudKitMetadata*

Give cloudd a moment to crash and relaunch, and all will be good.