If you search the forum (using the forum search feature) you’ll see that there are many complaints and discussions on the WD My Cloud being slow or sluggish at times with various methods used to try and fix it. It part it is due to the My Cloud scanning media files. A process which can take a very long time (days even) depending on the amount of data on the My Cloud. The other reason why the My Cloud is sluggish at times is due to the low powered processor used. When the My Cloud is performing multiple tasks at once it can sometime slow to a crawl. See the following two links for more on possible ways to fix the sluggishness.
https://community.wd.com/t/before-you-rma-your-device-lets-talk-about-your-problems/136430
https://community.wd.com/t/faq-twonky-dlna-media-server-setup-use/95373/39
