Who are you using for DNS?

https://www.google.com/search?q=how+to+set+dns+in+windows+10&client=firefox-b-1-d&ei=bRy3YbK1JNeeptQPvfqE6AY&oq=how+to+set+dns&gs_lcp=Cgdnd3Mtd2l6EAMYAjIFCAAQkQIyBQgAEJECMgUIABCRAjIFCAAQgAQyBQgAEIAEMgUIABCABDIFCAAQgAQyBQgAEIAEMgUIABCABDIFCAAQgAQ6BwgAEEcQsAM6DQguEMcBEK8BELADEEM6BwgAELADEEM6BAgAEEM6DgguEIAEELEDEMcBEKMCOgsIABCABBCxAxCDAToICAAQgAQQsQM6CgguEMcBENEDEEM6BAguEEM6CwguEIAEELEDEIMBOggIABCxAxCDAToGCAAQFhAeSgQIQRgASgQIRhgAUOckWK86YNJXaARwAngAgAGLAYgBxAqSAQQxNC4ymAEAoAEByAEKwAEB&sclient=gws-wiz

Is this with all search engines? What engine are you using?

ipconfig shows: Connection-specific DNS Suffix . : lan but there is no number for dns.

it seems it happens only with google search (have not tried others), but if I enter direct adress like www.something.com it opens right away.

You need to add something for DNS. it would help with performance is the provider is close to you. Try this tool and then pick a dns provider. I use Cloudflare 1.1.1.1 / 1.0.0.1, but you will have to find one that works best for your location.

Tool: https://www.grc.com/dns/benchmark.htm

thanks for advice. I've ran the benchmark, entered the dns values, but no result. First search is still idle for 30-40 s. Same with my laptop on the same network. (both wifi)

I will assume you also put the same DNS values in your router? Can you also download different version of Firefox just to test. https://www.mozilla.org/en-US/firefox/all/#product-desktop-developer

Also, what security software are you using? Did you ever run Firefox in Safe Mode and also try running Windows in Safe Mode to see if there is any difference. Can you also try plugging into your network to see if it makes a difference.