A380
Air Traffic Control Fully Certified
Training / Qualification; Situational Awareness
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance
Human Factors
I was working Assist 2 when training was occurring on Local Control 2.  Aircraft X took off and was climbing slow.  The Instructor told the Trainee when to say clear for takeoff to Aircraft Y.  Aircraft X stayed slow for a while and Aircraft Y was gaining on Aircraft X.  The Trainee told Aircraft Y to maintain 023 feet; but the pilot couldn't stop at that altitude so he was told to maintain 030 feet.  Aircraft X was off the scope but I was told they had around 6.5 miles instead of the 7 miles needed.  If I was working; I would have waited another 20 seconds before departing Aircraft Y.
I was the Developmental training on Local [Control] 2 during the event. Aircraft X departed Runway 24L and I told Aircraft Y 'At the pilot's discretion; RWY 24L LUAW; caution wake turbulence.' I crossed several aircraft down field and addressed aircraft on approach. My Trainer and I both intently watched Aircraft X's speed as they slowly accelerated offshore. We needed to wait until 7NM would exist between Aircraft X and Aircraft Y before I could say 'Cleared for takeoff.' We waited beyond the normal benchmark for obtaining this required 7NM because Aircraft X was slower than usual offshore. As the airspeed of Aircraft X increased to 170kts; I cleared Aircraft Y for takeoff; anticipating that Aircraft X's speed would continue to increase--it did not; and actually maintained 170 kts off the screen. When Aircraft Y tagged up on the radar; it appeared that less than the 7NM existed; and we would not be able to obtain it without a speed or altitude adjustment. I followed my Trainer's prompting and first restricted the airspeed of Aircraft Y; then the altitude. According to my Supervisor; we ended up with 6.5 NM of separation leading to a LoSS [Loss of Standard Separation].As a Developmental with approximately 10 hours on position; I would recommend more techniques being provided for dealing with wake turbulence and how to ensure you obtain the necessary intervals. I also know that more time on position will prevent this scenario from repeating itself.
LAX Tower controllers reported a loss of wake turbulence separation do to aircraft speed.