I own a minecraft server with my IRL friends using scalacube hosting. We using vanilla minecraft 1.16.5. As we were playing, we noticed that some of the spiders at night were spawning with swiftness. I have never seen this on any minecraft server I have hosted or a regular survival world. If I could have a brief explanation of what this could be, please let me know. 

(Update) They are still spawning. There is no pluggins on this server or anything. We noticed that it was only spiders that were spawning with potion effects. As of the 1.6 update. They made it so they could spawn with potion effects. But this is too consistent, please leave an explanation in the comments if you have one.