When you use a -db switch, it is expected that your database will be indexed. This is done using a makeblastdb command, and will create several files that will end in .p?? where ?? stands for two other letters. When searching against a large database, to gain speed you would want to index it and use a -db switch. You most likely have not done this step.

When using a -subject switch, the database need not be indexed. The result will be the same as when using the -db switch, but the search will be slower and this is not recommended for larger databases. It is typically used for comparing two sequences, or when the target database is relatively small.