Are they already so happy about the victory or is this behavior of the cheat program? I don’t believe in what they think over the simplest move.
I do not know what other players do. I pause because I
- have time on the clock,
- am (at Lichess classical 1590) not a strong player,
- am not a quick calculator,
- have insufficiently memorized some basic mating patterns, and
- do not wish to blunder.
If your opponent still has time left on his or her clock, do you know a reason he or she should not pause?
When an annoyed online opponent of mine (usually an opponent who is losing) demands that I move faster, I merely
- kill the chat so that I can finish the game in peace and
- block the opponent so that he or she need suffer no further annoyance in connection with me during future games.
Meanwhile, if my opponent prefers not to wait while I use the time on my clock to think, then he or she can always resign.
Admittedly, it is indeed slightly impolite to run one's clock down for the mere sake of running it down, but I seem to encounter this peculiar form of impoliteness in no more than five percent of all games. When I do encounter it, I relax, stretch my legs, review the game's earlier moves, read a nonchess book, catch up on the news, do chores, attend to children, etc., and don't let the play bother me much. Otherwise, the time on a player's clock is the player's to use as he or she sees fit. A player owes no explanation of a thinking pause, not even before delivering mate.
A sportsman should probably not presume to instruct an opponent regarding how fast to think.
As to whether cheating is occurring, I have no idea. I doubt that it happens much in my games. It isn't happening on my end; and on the other end, who knows? Insofar as cheaters are not especially targeting me and insofar as the online platform on which one plays makes a reasonable effort to detect and deter cheating, I have not found it profitable to worry too much about that.