Why so slow access ?

I wonder why site stopped responding super fast, what is causing this slow response ?

See here --> https://snag.gy/UVP5hl.jpg

I know it's not a server problem, there are other WP sites working ultra-fast, also this is a cloud linux server, and also it's totally empty. This must be any plugin doing this .... I also just installed defender plugin, maybe this will affect dramatically ?