It becomes very hard for the server if a problem has long time limit.
The mistake I made is that I thought that problem A is difficult so less people will submit it. So I kept the time limit 10 sec instead of asking the problemsetter to reduce judge data. But it did not work. People tried all sorts of methods to solve this problem.
When reply from the judge becomes slow people tend to submit the same code more than once and the judge becomes even slower. Let's see what we can do in future.
I got so much (uncountable

) emails during the contest that it was impossible for me to reply them quickly.