The "My Time" recommended value is between 5-10 seconds. Any lower value should work too, but has a higher risk to be late due to possible network lags outside of the sniper.
It's not clear what a lower value than 5 would do. Do you mean it could actually take longer than 5 seconds?
If network is slower than usual or eBay is very busy processing other things, it may take longer time for the sniper to submit your bid.
This is rare, but may happen. eBay is improving in this aspect, but they also are getting bigger at the same time.
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