Slow query execution in an empty table. (after deleting a large amount of inserts)

Question

I have a table in an oracle database with 15 fields. This table had 3500000 inserts. I deleted them all.

delete
from table

After that, whenever I execute a select statement
I get a very slow response (7 sec) even though the table is empty. I get a normal response only in the case that I search according to an indexed field.

Why?