Slow does not matter to games they use well under 1mbps. What is important is signal quality. The quality is mostly related to how much interference you have from outside sources and to some extent the signal strength.
The best and most expensive option tends to be PCI cards that have antenna cables allowing you to place the antenna on top of the computer. PCI cards in general transmit at the full legal power but because the antenna are located close to the case a lot of signal is absorbed or blocked. USB you can extend away from the case but it is much harder to find USB devices that have strong power output. Generally the ones with external antenna are better but it is hard to say small stick type devices can also have good...