A80 Controller described a near overtake event on final when the leading aircraft failed to comply with an assigned speed after accepting the clearance.

Narrative

I was working AR-O; and got Aircraft X from my Arrival Feeder. I cleared Aircraft X for the ILS 27L Approach from about 7 miles north of ZINTU. This approach has a way point that will allow aircraft approaching from the north to join the final from more of an angle. Having a large gap in front of Aircraft X; I instructed him to maintain 210 KTS until ANVAL; about a 9 mile final. I saw him slowing prior to the turn from CUMRA to YABBA. I re instructed him to maintain 210 to ANVAL. At YABBA I switched him to the Tower frequency since we were running monitored approached; and one more time; told him to maintain 210 to ANVAL. They acknowledged ALL clearances. This created an unexpected condition. Coming down the final; he was 40 KTS slower than other aircraft; including Aircraft Y behind him (cleared to maintain 210 to DOOOH; then 180 to DEPOT). I called my monitor; and he talked to Aircraft X; who said he wouldn't be able to do more than 180 KTS on the approach. This was unsafe. I had to slow the next 5 aircraft in a quick fashion (the monitor slowed the trailing aircraft and was a bare 3 miles between Aircraft X and Aircraft Y). I didn't mind if he didn't want to go fast down the final; but the crew needs to SHARE this IMPORTANT information! They aren't the only airplanes in the sky. It almost created a go-around scenario for the trailing aircraft; and it did cause unnecessary workload for my Feeder and me as I had to adjust for the surprise. I hope you can get this to Aircraft X and instruct the crew to COMMUNICATE so they don't cause an operational error by keeping secrets. Advise Aircraft X operations to find this crew and remind them to communicate with ATC; or DON'T ACCEPT THE CLEARANCE THREE TIMES if they have no intent to do any of it.