SQL & PL/SQL

Dear All,
I have the following problem of getting the output table from the input table below.
Do you have any idea of doing this with the help of analytic sql?
p.s. I have done this by using pure plsql block which is too slow to run with high amount of data. Below data is just a sample, in real I have millions rows of data.

Input table:
TIME USER VALUE

1 A X
2 A X
3 B Y
4 B Y
5 A X
5 B X
6 A Y
7 B Y
7 A Y



Output table:
START_TIME END_TIME USER VALUE
1 2 A X
5 5 A X
6 7 A Y
3 4 B Y
5 5 B X
7 7 B Y