Usually video players accumulate enough data in buffers to be able to play. The buffer size may vary, but it can be up to 10-30 seconds.
If you want a faster start, then have a look at streamable formats, such as HLS or MPEG-DASH. Some platforms support these formats natively, for example, HLS is supported on iOS and Android.
The idea is that HLS has references to multiple versions of your video with different bitrates and depending on your network conditions it may dynamically adjust quality to the network speed. Also, usually players start playback with the lowest bitrate, so it will need less data to download, and then gradually will switch to higher bitrates.
Apple has good tools and documentation about how to prepare videos for streaming.