Captchas are a big hurdle for people, especially impaired, while bots get better and better on them.
In fact that is the reason why re-captchas have changed their model away from capchas: 80% of their solved were done by bots, they estimated.
Anyway would be bad for the system, the big point is that it is easy to reward after all.
Same goes for minimum time open. It would first prevent voting from the first page (which can have different reasons like "I just closed the tab with the article and forgot to vote!!"). It would also not stop bots from voting. It would only slow them down to maybe 1 vote every 10 seconds while they load the next articles in other tabs.