mikrotik rb951g-2hnd speed problem

I have 40mbps internet speed but since I got my mikrotik rb951g-2hnd yesterday it went all the way down to 15mbps (wifi).
Before that, on old router I was getting roughly 35 mbps speed but with this new, even when standing right next to the router i get 15 and less.

Now, it wouldn’t be such a big problem if the speed was consistent, but it falls down to 5-4 a lot.

Is there anything I can do in winbox program to fix this?
I already changed my channel to a most unused one but it didn’t seem to help at all.

What are the best possible settings for this version?