On the Flight Schedules page on delta.com click on the Historical Reliability Information link.

From there, you can click on Searchable Database.

You can search by many different criteria:

Summary Statistics - All and late flights (total number, average departure delay, average taxi-out and average scheduled departure) and late flights (total and percent of diverted and cancelled flights).

Detailed Statistics - Departure and arrival statistics (scheduled departure time, actual departure time, scheduled elapse time, departure delay, wheels-off time and taxi-out time) by airport and airline; airborne time, cancellation and diversion by airport and airline.

The most recent data is FEB2008; searching DL4511 doesn't show CLE-ATL as a route during FEB2007-FEB2008, so searching orig airport CLE & destination airport ATL for that time period shows the percent of flights late:

DL 16.95
EV 31.86
OH 52.50
OO 11.25
XE 19.57