As a result of the union of Lithuania and Poland, has the situation of the Slavic people worsened?

Yes, it got worse. Before Lithuania became involved with Poland, the position of the Russians and the Lithuanians was the same, that is, they had equal rights. But after Lithuania became part of the Commonwealth, the exploitation of the Russian part of the population began. This was the reason for the outbreak of riots and flight. So at the end of the 16th century, the Cossacks arose, and the Zaporozhye Sich arose on the Dnieper River. In the future, the Cossacks became a big problem for the Commonwealth and its political integrity.