Actually, the amount of data will have no effect at all if you leave enough for normal operation (a couple gig).
This is because it's a solid state drive so it doesn't get fragmented at all, so files don't get spread out, and there's no degradation of performance whatsoever.
note: I assume you just are asking about the lag and not the space limitations, which are obvious (and un-upgradable, other than an external drive, or SD card; I use SD cards myself)
Really, with the cool external USB (soon thunderbolt) drives, storage options are more numerous today, although it's always nice to have everything onboard.