Time Machine takes way too long
It depends: 

TimeMachine on WiFi

TimeMachine does not backup if heavy traffic is going over the Network, there is something like Quality of Service running in the Background which throttles the upload speed to you TimeCapsule.
Also your WiFi Connection could be bad, as it was in my case. I did not recognise the bad connection with surfing (less upload and general only 8Mbit/s Speed) but 1,5GB to upload over a bad connection >1Mbit/s would take about 3 hours.

TimeMachine on USB

Is your drive full? Disks can operate in a performant way up to 80% of capacity, the they get significant slower. 

Is you drive old or defect? Sometimes drives are doing a lot of seeking and old drive have a high seek error rate, this will slowdown writing (especially at a >85% full disk an small files) 

Are the files small? Many small Files takes longer to get sorted and processed my the Time Machine. 

Are you using USB3? Make sure the drive is away from you Macbook, this helps sometimes with old USB3 Chipsets on external HDD enclosures. 

General Suggestions:


Turn on an Off you Mac
Make sure Hardware is okay (HDD test) 
Make sure connection to TimeMachine is good
Type in Terminal: killall mdworker
Try Again ;) 


Apples Suggestions:

Slow Time Machine

Try these suggestions if Time Machine is slow.
The first time you use Time Machine, set it up in the evening so that the initial backup can be done overnight.

If your backup disk is a Time Capsule, leave your Mac in the same room as the Time Capsule for the initial backup, or use an Ethernet cable to connect your Mac to one of the Ethernet ports on the Time Capsule.

Virus scanning software can make Time Machine backups very slow. If you use Norton AntiVirus or a similar product, try to exclude your backup disk from automatic scanning. Also make sure you are running the latest version of the virus scanning software.
 Exported from Apple Support Site

OS X Yosemite: If Time Machine remains in the “preparing” stage

After the initial backup, Time Machine backs up only files and folders that have changed since your last backup. Time Machine displays a “Preparing...” message while it compares your current files and folders to those in the most recent backup.
The length of preparation time Time Machine requires depends on the amount of information on your Mac, the size of the backup, and the speed of the connection between your Mac and the backup disk.


The preparation stage can also take longer if:
The previous Time Machine backup was interrupted.
The Mac wasn’t shut down properly.
A disk was disconnected from your computer before being ejected.
A software update was recently installed.


You used your Mac for several days without performing a backup. This can happen if the backup disk was disconnected.

Antivirus software can also cause backups to require more time. Make sure your Time Machine backup folder, located on the backup disk and named “Backups.backupdb,” is excluded from virus scanning. Exported from this Apple Support Page

Last but brutal tip:

Reformat the TimeCapsule or the TimeMachine drive and make a new initial Backup over Ethernet or USB. And restart you Macbook, sometimes the mdworker is slowing down.