I disagree. It's easy enough to get distracted or fatigued during a long session of solves, and that wouldn't be indicative of a competition setup either. (Unless you're the kind of person who participates in almost every event!) At a comp, you do only five solves per round and there's a long gap between rounds (if there are multiple rounds), which provides plenty of time for you to calm your nerves.
Bad practice is worse than no practice at all. If you feel like your times are consistently getting worse even though you're not getting bad cases (nb: this is subjective), then just take a break. Go do something else for an hour or two.
If that isn't normal,
lol.