You sound nasally, try and take pauses and breathe more...
Firstly slow down for a while, it is possible to spit your lyrics slower and still on beat. You're focusing on getting the words out as opposed to emphasising each syllable with your lips and tongue, that's why you're described as mumbly. I'm one of those people who can't understand what you're saying and I listen to fast MCs. One trick to articulation is in the lyrics them selves, write lyrics that kick on each syllable, slick, sick, trick, pick, quick, flick, etc. Bu and Ku and Du and Gu, It and Mit and Tit and Git. I don't know the technical terms, but these sounds have bite to them, they're very clearly spoken it's hard to make these sounds without people understanding what you're saying. Understanding the sounds of each letter and word and using them well in your lyrics automatically makes what you're saying more articulated.
This one is a bit controversial but it's quite simple, listen to and copy over the top of other MCs who have good articulation. The problem with copying is that you imitate people's style and pick up bad habits, but at the same time you learn to use and apply syllables to beats at the same level of articulation as they do. It's then up to you to write your own lyrics and take the skills you've learned from them, and eventually you'll develop the instinct to just know how many syllables fit the beats and what words you can use to fill that space.
The obvious stuff is just to say that what is important is that people hear the words and understand what you say, as opposed to how many words you try and cram into a beat. There's nothing to be gained from hearing that but it's an attitude you can adopt.