Why do I get serious execution lag on clicks, application switching and general usage
Let me explain to you how RAM works. Data is loaded into RAM first, in blocks called "pages". That is called a “Page in”. When a piece of memory is just paged in, it is in Active or Wired Memory. Wired memory is the memory used by the Core System and Active Memory is memory used by other applications. 

When a page in Active Memory is no longer being used, it becomes Inactive Memory. The data still stays in RAM because an app may request it later, at which point it becomes Active Memory again. 

When your RAM starts to fill up, the oldest pages in the Inactive Memory region are “Paged out”, which means that they are written to the Swap. Your system is slowing down because of this. Your RAM is full, so the system is paging out things to the disk. The disk is slower than the RAM, so your system slows down as data is paged in and out. 

You must find out the app(s) using up your RAM with a combination of trial and error and using the Activity Monitor to check the memory usage of each app. Most likely, the problem is a memory leak in an app.