iOS cannot play "slow" or "veryslow" h.264 videos
Use next parameters to ensure compatibility with apple devices.

-profile:v baseline -level 3.0

This disables some advanced features but provides for better compatibility. Typically you may not need this setting (and therefore avoid using -profile:v and -level), but if you do use this setting it may increase the bit rate quite a bit compared to what is needed to achieve the same quality in higher profiles.

Encoding video using H.264 compression:


H.264 Baseline 3.0: All devices
H.264 Baseline 3.1: iPhone 3G and later, and iPod touch 2nd generation and later.
H.264 Main profile 3.1: iPad (all versions), Apple TV 2 and later, and iPhone 4 and later.
H.264 Main Profile 4.0: Apple TV 3 and later, iPad 2 and later, and iPhone 4S and later
H.264 High Profile 4.0: Apple TV 3 and later, iPad 2 and later, and iPhone 4S and later.
H.264 High Profile 4.1: iPad 2 and later and iPhone 4S and later.


A frame rate of 10 fps is recommended for video streams under 200 kbps. For video streams under 300 kbps, a frame rate of 12 to 15 fps is recommended. For all other streams, a frame rate of 29.97 is recommended.

Encode audio as either of the following:


HE-AAC or AAC-LC, stereo
MP3 (MPEG-1 Audio Layer 3), stereo