You know how it feels. You learn a fast tune – a reel or a jig. You can hear it in your head – faster, faster, faster! Sadly, your fingers are not q-u-i-t-e ready for that! But we’re focused and so we push. We push the tempo so we can play as fast as everyone else. We want to play faster than everyone else!

So, we set ourselves the task of speed. And we repeat (a lot, as we talked about here). We push as hard as we can.
AND IT ALL FALLS APART. Why (whine)?
This happens because we are focused on the wrong thing. No tune is good if it isn’t accurate and well played. That’s true of airs but one could argue that it’s even more true in the fast tunes. In a slow air you have time to fix things in real time. But in a reel there’s no buffer – there’s only hanging on and hoping for the best! And that’s stressful.
Stress is not good for fast tunes. It seizes your hands, your arms, you brain! You need to r-e-l-a-x!
No really. Try playing some tune you think you know as fast as you possibly can. When you’re in the middle of the tune, stop playing and check-in with yourself. Is your hand relaxed? Arms? Shoulders? Butt? Legs? (no, really, check your legs too!)
As long as you are holding a lot of tension and stress, you will not be able to play your fastest. Sorry, but it’s true. You have GOT TO RELAX!
And to get relaxed, you have to play at a tempo that doesn’t stress you out! If that’s at Lento/40, so be it! There’s no shame in a well played tune at an approachable tempo. Only when that feels easy and all the notes are correct (no, really – all the notes, rhythms, harmonies, etc.) will you be ready. Then you can speed up a little (like to 44!).
Use the “feels easy” and accuracy as metrics for when you might try going a little faster. This is where you can push yourself a little – each time it feels easy and is accurate (and be very strict on this – accurate is accurate, not sorta accurate, not mostly accurate – actually accurate!), then go a little faster. If you like more precision, use your metronome for each speed and make it part of your accuracy metric. When you hit your limit (and you will – we all do), slow down a little bit and get accurate again, then run at it again. This is a process that will take a little time but will help you develop a tune you thought was cool enough to learn into a tune you love to play!
Give it a try – take on a new fast tune and work it up this way. Let me know how it goes (in the comments!). Don’t have a fast tune – you can always ask me!