Spell speed reduce ur RECAST time aswell... Light speed gives 4 casts with 0 spell speed but u can Gain a 5th cast by stacking spell speed... Dont belive me try it xD

If u did not know this then all i can say is wow talk about an over sight
Its near around 900 spell speed u get the 5th cast (can almost get a 6th with arrow) and i knew this by self testing as i was curious if i could get another cast by reducing my RECAST time

And yes its true that lightspeed only reduce ur cast time only and not recast... But spell speed reduce cast and recast which benefits lightspeed a lot