Plugin Version

[21:58:51 INFO]: ChestShop version 3.9.3-1.12 (build 20)

Plugin Config

https://pastebin.com/e8TaNHWQ

Server Version

[21:59:38 INFO]: This server is running Paper version git-Paper-1618 (MC: 1.12.2) (Implementing API version 1.12.2-R0.1-SNAPSHOT)

What is happening?

I have lag issues with ChestShop
https://spark.lucko.me/#ffr3R14Ich

Spark doesn't really point to any issues as far as I can tell, main reason I see in it why it might be slow-ish is because of the economy plugin used. I suggest limiting how fast a player can interact with shop signs by increasing the SHOP_INTERACTION_INTERVAL if you are having issue when they do it too fast.

Which economy plugin do you recommend?

I personally use CraftConomy3 but I doubt that changing your economy plugin will actually "fix" anything. Any economy transaction will have a small impact on the performance which increases the more transactions happen at the same time.