Just a quick calc moving the speed of Leona's Solar Flare attack into an actual blog instead of a short calc on her page.
The description of Leona's Solar Flare ability states that she is able to call down a ray of solar energy that reaches its destination after .625 seconds. Considering Runterra is likely similar to Earth in its distance from the Sun, that would make the distance between Runeterra and the Sun approximately 149597870.7 kilometers, or 149597870700 meters. Leona's attack crosses this distance in .625 seconds.
149597870700 m / .625 s = 239356593120 m/s
239356593120 m/s / 299792458 m/s = 798
For the attack to travel from the sun to Runeterra in .625 seconds it would have to be traveling approximately 798c, MFTL.
Buuuuuuut i'll just do a lil' calc of the attack coming from the sky, just in case...
Start of the Stratosphere = 18000 m
Start of the Mesosphere = 55000 m
Start of the Thermosphere = 107500 m
Start of the Exosphere = 600000 m
18000 / .625 = 28800 m/s, Mach 84
55000 / .625 = 88000 m/s, Mach 258
107500 / .625 = 172000 m/s, Mach 504
600000 / .625 = 960000 m/s, Mach 2818
MFTL from the sun and HH+ to MHS+ from the sky.