One problem that you may encounter when using your Mac is pauses when presented with an "Open" or "Save" dialogue box. While the foremost application may be running just fine, when you try to access the save or open dialogue box there may be a long pause that sometimes is accompanied by the spinning color wheel. This is usually followed by normal functionality, but sometimes the whole system may be slow when the box is open.
We covered one reason for this problem in a, which involved rebuilding the launchservices. However, there are are a couple of other potential causes, which range from it being the normal behavior of the Finder to various cache and hardware issues. Some people have fixed their problems by changing settings in OS X, but others have had to update software or firmware.
Generally, pauses during open and save dialogue boxes happen because the system is unable to communicate with the filesystem. The save dialogue box queries all available filesystems and waits for them to be ready before it will allow you to navigate through folders and write to them. Because of this, anything that can cause a delay in filesystem access will result in a pause, which depending on the cause can be quite long. One drawback in the Finder is that it may wait for a filesystem even though you have not navigated to it, and is one of several reasons why these delays would happen. While some may question whether or not Snow Leopard is to blame, a few people who downgraded to Leopard still experienced the pauses, indicating in their situation the system software configuration was not the problem.
Optical Drive Readiness
If you have a CD or DVD in the optical drive, the system may spin it down after some time to preserve power. This may cause the Finder to pause when when you invoke an open or save dialogue box if the drive has been queried. The only way to get around this problem is to use a utility to prevent the optical drive from spinning down, such as "SpindownHD" which is included with Apple's developer tools (free from the developer connection site).
This may also be the case for both internal and external hard drives, which may go to sleep mode if not used. One way to tackle this is to uncheck the box "Put the hard disk(s) to sleep when possible" in the "Energy Saver" system preferences, which should keep both internal and external drives from being spun down. Some drives may interface with special controller software, so if you have that software in your setup you may consider reinstalling or updating the software. Many people with this problem have been using a Drobo RAID system, and upon updating the "Drobo Dashboard" software the problem disappeared.
Corrupt filesystem and partition formatting
If a mounted filesystem (especially the boot drive) is corrupted, it may result in slow filesystem performance. While the computer should show pauses in general if this is the case, there may be instances where an application loaded in RAM will perform just fine but upon saving or opening files the system will pause for a while.
While you can try using disk repair utilities (ie, Disk Utility or Drive Genius) to check and repair filesystems, the best remedy is to backup the filesystem by cloning or using Time Machine, both repartition and format it, and then restore the backup to it. The key thing to remember is to partition the drive, since just formatting it may not correct any problems with the partition table. Be sure you have Windows partitions cloned or otherwise backed up before doing this.
Along similar lines of partition problems is a recent issue with some hard drives in MacBook, iMac, and Mac Mini systems, where the drive would randomly go to standby mode and seat the drive heads. If the system has this problem it should occur at any time, but would also occur when open and save dialogue boxes are open. The problem only happened in a few systems, and was addressed by Apple with the "Performance Update 1.0" that was released last October. If you have not applied this update and have a MacBook, iMac, or Mac Mini from 2008 or later, be sure to install it.
This problem could be from software issues as well. While we previously covered rebuilding the system's launch services as a possible remedy, you might also try running a system maintenance tool (OnyX, Cocktail, MainMenu, etc) and perform a cleaning of all available caches and run permissions fixes on the boot drive among other available maintenance routines. Do this when booted into Safe Mode (hold shift at bootup) for the added benefit of running the system only with essential kernel extensions, which will prevent as much interference from background processes as possible during the maintenance tasks.
With the caches cleared, the next step would be to disable Finder-specific features that may use the filesystem when the open and save dialogue boxes are presented. These include disabling the smart folders in the side-bar, and deleting the spotlight indexes for local hard drives. The smart folder settings are available in the Finder preferences, and while the removal of the spotlight index can be done with a variety of utilities and terminal commands, you can easily do this by dragging the hard disks to the "Privacy" list in the spotlight system preferences and then removing them from the list.
Lastly, if you have mounted network shares, a delay or loss in the network connection may cause the Finder to pause while it tries to load the network resources. This may include iDisks, Time Capsule devices, and other computers on the local network. Try using the open and save dialogue boxes with the filesystems mounted and unmounted to see if they are contributing to the errors.