Typing and UI lag, yosemite

Hi,

I have a macbook pro 13" mid 2009 with SSD and 8GB RAM. It runs Yosemite 10.10.5.

When I changed the original HD with an SSD and installed Yosemite (a few months ago), the computer started working very well: fast, responsive, I was very happy about it.

Since a couple of weeks, however, I've experienced a very annoying typing and user interface lag, which I can notice in particular if I connect an external monitor 1920 x 1080 (HP 2311x).

In particular, the situation where I notice the problem more is when I use BBeddit and I maximize the edit window: if I type very fast, the characters appear on the screen even a couple of second after I have finished. With a smaller window, the problem is reduced. With Textedit instead, even if it's not very fluid, there is almost no lag.

I can notice some lag even when I resize a window dragging a corner (any window, not just BBedit windows) and when I switch from an application to another.

I've already tried several hints I got browsing this community:

- I've disabled textexpander

- I've deleted the com.apple.windowserver*.plist files

- Pram reset

But nothing has changed.

Thanks.