you can import any video and use the video plugin .h264 to play it. no need to convert it.
However when you play videos using the .h264 you need to take in calculation the platform users will use and if that platform is compatible with the codec and the video format. for example playing webm files wont be compatible with safari and apple devices. jus stick to mp4 files or avi
if the files are to big, just reduce the bitrate of the video in a video editor (Premiere for example.) usually a min 1Mbps/2Mbps Max bitrate video is usually enough for majority of files, keeping the video quality intact and frame rate( unless you are trying to use some 4k or 1080p complex video then quality is noticeable. When i say complex i mean lots of stuff going on in it. if the video is a simple video, like a man talking with no action in background or 1 object moving with not much of environmental lights moving around, then your safe reducing the bitrate.)
example of same file with reduced bitrate 1-2 MBps bitrate 1080p quality same in this one being a simple video ... white background 1 colored character around moving. if we where to add some crazy lights then frame rate would skip and pixelation would be a lot more noticeable. <- size 20Mb
original file at 10-12 Mbps full 1080p <- size 40 Mb
but this is a .mp4 example .avi has more weight... the .mov files are as close as light as a webm just they dont play well with devices unless you got quicktime installed. .webm files are mostly incompatible with some of the apple devices as mentioned above.
size of the same video above on all formats would be around :
.webm 10-20 mb -noticeable behavior of .webm files played in firefox... they are flipped on vertical axes for some reason and sometimes take 1 to 2 seconds to start playing.
.mp4 20-40 mb
.avi 100-200 mb
.mov 40-80 mb