We understand your concern.
However, we also have coders from Div 3 & 4 participating in these contests. Since they are at a beginner stage, they need more time to solve the questions, and hence the 3-hr duration. Having said that, we do have a monthly rated contest - Cook-Off, the duration of which is 2.5 hours.
Regarding the plagiarism concern - we are actively running plagiarism checks for all our rated contests. However, the process is time taking, so we request your patience. If you come across any specific cases, please mail them to help@codechef.com and we’ll surely take a look. You can also catch our updates on the no. of cheaters in every contest here: Updates on curtailing plagiarism