It's not just write speeds. Read speeds vary wildly too, and they're not generally marked anywhere. For the best read speeds, you either want to research individual card models (and stay with name brands, because off-brands -- even relatively well-known ones -- frequently vary wildly from card to card of the same model, because they use whatever controller etc. is cheapest on the day), or you want to go for cards with high write speeds (which are generally faster for reads too).
Either way, go major name brand if you want speed. And buy from somewhere well-trusted -- Amazon direct (not a third party selling through Amazon), a major retail chain, etc. Flash cards are one of the most common counterfeit products around, because they're tiny, a standard size / shape, and extremely easy to smuggle.