Just for your information, lag and low fps are two different issues. Most times the lag can be attributed to client-side network connection issues. However its fairly obvious that during these events especially when there are many people online and in-game during peak hours the latency between the server and the clients increases thus causing many players to experience lag.
When you get frames per second drops, your computer is failing to properly render the game optimally. Most times this is because the graphics processing unit (GPU aka video card) is unable to render the game at high graphic / video settings i.e resolution etc..Your system components may also be the culprits like your CPU, Harddrive, RAM which can bottleneck the fps within your games. The most obvious sigh of FPS drops is you get a slideshow like effect within game. The issue you're experiencing is server lag. Which is obvious by the delayed response with input action / feedback. i.e you set an ammo spike and nothing happens until a few seconds later.
Here's an article that further clarifies this :
http://www.howtogeek.com/142193/htg-...he-same-thing/
edited to add - Google search FPS and Lag if you want even more info