It causes all servers to lag, just not typically a noticeable amount, seeing as how most servers have many more plugins that are just as efficient or less efficient. There are far too many variables to determine just how much latency it causes exactly, due to different player counts, different commands involving more code than others, server hardware, etc.
Would your server continue to lagg more as you add plugins even if you have a overkill pc? Wouldn't 1 plugin be the same as 20 on capable enough hardware or is it a software issue
It depends how many players you have. The more players registered on your server, the laggier economyapi becomes.
That makes sense (I have around 45-50 players with no noticeable difference between 1 player and a full server)