DS920+ sometimes very slow at just opening photos?

Hello,

I bought my first Synology NAS around 6 months ago. I stored most of my pictures that are important to me on the NAS, but when I try to open them, sometimes it goes reaaaly slow. Around 1-2 minutes, just to open 1 photo. I tried to move a few photo's and halfway the transfer just stopped.

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However, a lot of the times opening photo's is super quick and moving photo's is almost instantanious.

I don't understand the change in speed. Does someone know why there's such a big difference? I can imagine if someone is doing something on the internet in my house the speed might drop a little, but not drop to 0, doing nothing.