Thank you for your replies. I have installed it AFTER I deleted everything (data, etc), like a new ipad. Overall it has very good performance but some games have a little bit of lag. For example previously in "Game of Life" with ios 11 in the start where it shows the map rotating the camera was smooth, now it has some slow downs. However it is not so important. Other games are like before, eg Horizon Chase, Smash Hit, Kingdom Rush series, etc. Of course I didn't regret it. The OS is now way better and we have apple arcade, something that we couldn't have before, along with PS4 / XONE controller!
Programs have the same performance too. I wish they will add some day a button that will close ALL open apps like android has for years!