I have 2 doubts without any linking

1- I have a veeeery old pc and I'm using fullscreen mode because someone told me that fullscreen = better in game performance, while windowed mode = worst performance, but quicker for alt+tab issue. Does it have any clue? Or is it maybe the opposite ?

2- I tested in Uthgard the spiritmaster pets intercept rate. I'm sure that lower the lv pet is, higher is the intercept rate (lv 7 pet > lv 12 > lv 20 > lv 32).
Is it correct that a sm have to chose between the procs and the higher intercept rate?

1 just try it. for me it runs smooth in fullscreen and lags in windowed mode.
dunno why it is like that,but maybe it is vice versa for you.
2 somebody reported it in the german section . think it was holunderman

1) As a general rule what you have stated is true but as Salbei says try different options

2) I noticed my low level pet intercepted at an insane rate compared to my higher level pets, but I put this down to lower level pet dex buff in the summoning spec line, I'm full suppression spec. From memory the low level sm pet intercepts a lot more frequently on uthgard then it did on goa/mythic servers.