You know that fast, free inflight Wi-Fi your airline promised? If you've been on a flight lately and found it to be ... well, kind of slow ... you're not alone.The Wi-Fi provider that supplies on-board internet to several of the nation's biggest airlines admits its service has been a bit sluggish recently as more customers than ever log on, stream videos and eat up precious bandwidth.Elsewhere, passengers taking long flights to Europe and Asia h…