El Capitan Time Machine on NAS running very slowly?

So I upgraded my Macs to El Capitan and everything was rosy - with the exception of Time Machine.

Time machine backups to local USB drives (Seagate 2TB if it makes any difference . .) seemed to work fine, but network backups to a Synology DS415Play NAS were getting 'stuck' on the 1st few MB - and staying stuck, seemingly forever.

The symptoms were visible in the 'progress' of Time Machine in the status bar:

The 'backing up' would stay in single megabytes.

After checking the network etc, I then tried Time Machine preferences:

Originally I had my NAS time machine connected as the 'auto discovered name' i.e 'AndyNAS'.

I tried the 'add or remove backup disk' option to remove the connection and re-add it. It didn't seem to help - I got the same problem.

I tried connecting directly to the Time Machine drive using Finder and for no reason that I could think of, used the numeric IP address to connect to the NAS rather than the name. This seemed to work fine and the Time Machine share was visible and seemed to be working normally.

Finder->Go->Connect To Server

In a last ditch attempt, as I have my NAS on a static IP on my LAN (192.168.0.100), I tried removing the original Time Machine NAS connection using 'Add or Remove Backup Disk' and then tried connecting using the 'dotted quad' 'numeric' IP address as shown above - and everything started working again!

Not sure exactly what fixed it - could be any of the following:

• Adding and Removing the Backup Disk / NAS connection in Time Machine Preferences
• Doing a direct afp connect to the NAS Time Machine share in Finder
• Using the 'dotted quad' / Numeric IP address rather than the name
• Pure luck  and the 'turn it off then on again' effect - but it did fix 3 out of 3 of the macs I tried it on

To be fair, I'm not that bothered - it's working again, and for all three Macs (work, personal and wife's) backing up via Time Machine to the NAS after the El Capitan upgrade. It's also encrypting fine.

As a side note, I didn't have to re-create my Time Machine backups - as long as I re-entered the encryption password then it 'found' the old backups and carried on as before.

As usual, this worked for me - your mileage may vary but it might be worth a try.

***Additional***

After an interesting dialog box of 'Time machine has checked your backup - it needs to backup from scratch' (effectively) I found that having to back up 150GB or so was taking a long time.

If you check 'console' and search for 'mdworker', you might see a number of entries, especially starting with the word 'deny'. Apparently, this isn't helping your time machine speed!

Believe it or not, the fix for this is to start your Mac in 'safe mode' - then restart again normally.

To start in safe mode, shutdown your Mac. Then start it up. Immediately after the first 'bong', hold down the shift key. When you see the startup screen, you can release the shift key. If you're successful, you'll see 'safe mode' written on the screen somewhere.

(Apple guidance here https://support.apple.com/en-gb/HT201262)

After you've started up in safe mode, give it a minute or two to finish loading everything then shut down again.

Now check your time-machine backup speed - assuming you have a decent network speed (I highly recommend gigabit) you'll see it fly along . . .