I have been getting numerous reports about issues while sending a resource pack to the client regarding a client freeze and how I could or should fix it so here is some info about it and the explanation why I cannot do anything about that from the server/plugin side:

When a server resource pack is send to a client it will shortly freeze after the download is complete. (Same as manually applying a local pack) This happens because the client has to reload all resources, check which resources overlap and do some other calculation and manipulation with them. (Like stitching all the textures together to a single image like they were in the first texture pack format) It's not a download issue as the client will locally cache the pack files and not re-download them if their hash didn't change.

How long this takes generally depends on the PC's performance and the size of the textures (HD packs take longer) that are used in all resource packs that the player has applied. (So even if the server resource pack doesn't contain any HD textures or textures at all the client will still completely reload all locally applied resource packs) This is something that has to be fixed in the client by Mojang for example by only reloading resources that changed. There's not a lot I can do about that other than advice people to put pressure on Mojang to improve that behaviour.