Also this wasn't the first time Slade Beat down the JLA. He whooped Hal Jordan and Aquaman's ass simultaniously in one fight. Later that same issue the Flash catches him out in the street and pretty mcuh beats the piss out of him. Even taunting him by saying "Come on Slade you didn't really think you were faster than me?" When he is about to be handed to the cops he tosses a cop at wally and takes off then wally chases him. Slade had ducked around a corner and stuck his staff out and tripped Wally seinding him flying inot a building.
Kyle did a lot better than Hal Jordan did against Slade at least Kyle made him work for it.
Slade has been whooping Wally's ass since he was a teenager, so I don't know why anyone is surprised. Wally needs space to get up to light speed anyway and he has to be running in prettymuch a straight line. Slades explosvie mad him run in a cirlce casuing him to slow down and each explosing casued him to change direction making him slow down evne further, right down to about Kid Flash levels and we all know what Deathstroke does to Kid Flash.