It is surprising to hear that their performance is declining. Personally, I haven't noticed any such thing. Messi has scored frequently for his new club (Paris Saint-Germain), while Cristiano Ronaldo is in good form for Manchester United. And recently both the players have helped the national teams in various tournaments. That said, I would not make a comparison between these two players. Both of them have their own unique set of skills. In some domain, one will be better than the other and the reverse may be true in some of the other domains.
There is no doubt now this is happening because age factor is going to have big impact on them as well with Ronaldo is 37 and Messi is 35 which mean they are now not good as we have them in last world cup 2018 they already both are over 35+ which mean they can't play like they have done in previous events.
Even both are giving their best in club games Messi already done good in new season in PSG, but Ronaldo is unable to perform as he is currently under some good rumors for change of club because of his desire for Champions League, but both are capable of giving their best in this world cup which could be last for these both players and many more which are already near them in age factor.