My question: is there any difference to forum if Ignorelist is empty or full of members, I mean does it have any effect on browsing speed, lags, opening pages, etc?
From my personal experience, having so many members in your ignore list generally speaking, does not affect your browsing speed, and it also does not cause lag or slows down pages from opening.
But....
Your ignore list page (I believe) is the page that is affect, having so many members listed on that page will definitely cause the page to be very slow in opening as loading and bring up all the users listed will take some time, it might also cause your browser to lag, most especially if you are on mobile.