TimeMachine

From MacFixIt

Possible reasons and fixes for Time Machine slowdowns

Time Machine has been a great backup solution that has for the most part worked quite well; however it has not been without its glitches. Despite the majority of initial problems being addressed by Apple, there will be times when the backup process seems to just hang or stall out, without giving any apparent errors or notifications.

To easily check out Time Machine logs and track down problems and exactly what Time Machine is doing, download the "Time Machine Buddy" widget, which will display the TM logs in the dashboard.

Hanging may also prevent other operating system functionality, causing spinning beachballs and the inability to shut down when the backups are going. This type of behavior usually indicates the system resources are not accessible to the system when the system is expecting them to be. For instance, if an external drive is not responding as it should, the system may hang while it waits for a response from the drive. There are several reasons why a Time Machine drive may suddenly be inaccessible:

Drive malfunction.
If the drive is making clicking sounds, or doesn't appear properly either on the Desktop or in Disk Utility, then there may be a problem with the drive. For external drives, low power to the drive can cause the system to hang.
Volume corruption.
While the disk may be fine, if the formatting or partitioning of the drive has problems, then the drive will not be properly accessible by the system.
Drives being put to sleep.
The energy-saver setting to put drives to sleep whenever possible can cause them to go into a state where they won't wake up properly. This depends on the drive itself, but while the system waits for the drive, you can experience a hang.
For drive errors, you can try running Disk Utility to check for and repair errors on the drive, or you can more thoroughly do this with third-party disk utility software such as "Disk Warrior," "Drive Genius," "Disk Tools Pro," or "Tech Tool Pro." Checking both the boot drive and the Time Machine drive for errors is recommended.

Beyond drive-specific issues, there can be incompatibilities both with other system resources and third-party applications, which can cause Time Machine slowdowns. If you have antivirus software running, many times this can interfere with Time Machine's functions, especially if you have live scanning or "on-access" scanning enabled. Turning off these settings in the antivirus software may help this situation. Additionally, if you have spotlight enabled for the Time Machine drive (it is enabled by default), it can sometimes endlessly try to index the drive. As such, you can try adding the Time Machine drive to Spotlight's privacy list, and then remove it to restart indexing.

A similar thing can also be done in the terminal, which will ensure the spotlight stores are deleted and started anew:

Open Terminal
Type the following command and include a space after the command:
mdutil -E -i off
Drag the Time Machine disk to the Terminal window to enter the full path to the disk, such as the following:
mdutil -E -i off /Volumes/TMDisk/
Ensuring a space is between the "off" and the drive path, press enter
Repeat this command, changing the "off" to "on" in order to enable spotlight on the drive again.
Finally, if you are backing up over a network (especially a wireless network), backups can be slow by nature. At 54Mb, speeds of most wireless connections, you will run at a maximum of 6MB per second, which translates to 14 hours for a 300GB backup when running at optimal conditions. Given network overhead and other interferences, this can easily double and result in the backup taking a day or two. For the initial backup to a networked device, you might try plugging in the Ethernet connection, which should be at least double the speed, but up to 20 times faster than wireless.

An additional fix for slow backups can be to restart Time Machine on the drive by removing it and re-adding it in the Time Machine preferences. Doing this seems to clear various bottlenecks in Time Machine and start backups running at faster speeds again.