Sorry for the late response but there won't be an fix until the next WARNO update. Eugen Systems (the game dev) restructured the game files responsible for Decks and Battle-groups.

An unintentional side-effect was that workshop mods with freedom decks containing all or most of the units in the game now cause a massive lag spike in the game menu.

Currently, all I can do is lower the unlimited deck card amount from 999 to 25. This is a temporary fix and I will revert the cards back to 999 when Eugen releases the next game update.

For now, you can also refrain from saving unlimited decks and only use the limited ones for now. This should prevent the lag issues you're experiencing until then.

I am very sorry and I wish there was more that I could do

Locking this thread as this has been fixed with the new game update