I think it's FLV or H.264, but I'm not sure.
FLV is a container format. H.264 is a video codec. Those two are not the same. h.264 is typically stored inside an FLV container.1 Therefore, any video you see on YouTube is offered to you as a FLV (Flash Video).2
YouTube by default stores their videos internally as h.264.3 That means any h.264-capable container mentioned in YouTube's supported upload formats is fine. I guess if you upload FLV files, or even WebM-encoded files, they shouldn't be transcoded, but stored as-is.
Does my video load faster if I upload to youtube a video with lower bitrate? (Given the same resolution)
Generally, yes. This assumption only holds true if there are no differences in available bandwidth from YouTube itself – their servers might be overloaded for some videos – and YouTube does not re-encode your videos.
1 – See here for more info: What is a Codec (e.g. DivX?), and how does it differ from a File Format (e.g. MPG)?
2 – unless you use the HTML5 feature, which would serve it without a Flash object
3 – according to Wikipedia