Reboot and Shut down taking ages

I’m running Kodi 17 on Raspberry Pi 3.
I recently switched all my media to a Synology NAS (instead of a plugged in external hard drive).

Since making the switch, whenever I reboot it either stalls, lags, bugs out or takes an age to reboot (or indeed shut down).

This causes me to have to power off the pi at the switch.

Any ideas what’s causing this?

Could it be a power supply issue?