Huge variability in blastn speed between same short sequence job
First of all, if you're using an online service like NCBI's BLAST, then the time to completion will depend on the load the service is experiencing. Your job is ut in a queue and will be started when it's its turn. If the service is under heavy load, then its turn might come later than you expect. Then, if the service is under heavy load, even after launching your job, it might take longer to finish because it won't have enough resources to run quickly. 

Since you say that the same job finished quickly on a new tab, it sounds like either the first job was last in the queue or there was just a bug that caused it not be launched or something like that. We can't really help with things that resolve themselves like that.