The only place so far where you will see any slowdown, if indeed there is any at all, will be the Castlevania intro.
But seriously, even if there's a hiccup, exactly how often are you going to view that intro? Knowing that your hardware hiccups on an intro sequence is hardly going to ruin your enjoyment of the rest of the 99.99999% of the DS gaming world which run perfectly on the SuperCard. You catch my drift?
I guarantee that you will get far more benefit from a 2GB card than a fast 1GB card.
But in any case I have the 2GB SanDisk and it's fine. I don't know about the Castlevania intro, but I really couldn't be bothered as I hate that game.
And don't worry too much about fakes. There are very few factories which produce MicroSD cards, and if they have the know-how to churn out a 2GB one, then chances are it's the exact same card as the standard SanDisk 2GB but with different branding. Just make sure you don't pay a premium and you'll be fine.