Understanding time format of the EXPLAIN command - Postgres
actual time=8163.890..8163.893 means

Initializing that step ("startup") took 8163.890ms 
Running the whole step took 8163.893ms

So in that case nearly the complete work was done in the startup phase of that step.

Edit:
The same logic is "applied" to the cost information

cost=2928781.21..2929243.02 means:

The cost to initialize this step was estimated at: 2928781.21
The cost to perform the step was estimated at: 2929243.02

(note that "cost" does not have a unit - it's an arbitrary value)

This is also explained here:
http://www.postgresql.org/docs/current/static/using-explain.html