Just got to 74k length (being able to encircle 15k snakes without them noticing is pretty hilarious), and here are my tips:
There is a quality button on the top right hand corner of the start menu, and low quality makes the game run smoother. It took me a very long time to notice it.
Small snakes drop more length when they die then they are long. Even 10-20 long snakes drop about 50 length.
When you go straight the kinks in your tail slowly straighten out (like a noose). For the most part this is completely irrelevant. When it isn't irrelevant is when you encircle someone with a lot of spare length left over. Go straight then turn around, and you shrink the circle that they have to hide in. Do this multiple times from different directions and you can kill them no matter how well they stick to the edges. Just make sure that you don't go too far out and accidentally let them out.
This also has the benefit of making it harder to encircle you.
Screens are wider then they are tall, this makes it safer to move (and sprint) sideways then vertically.
Early game:
Scavenge or kill snakes around your size. You can afford to be reckless.
Don't be too greedy though. Better to get 25% of a dead snake and survive then get 50% and die.
5k-10k size:
Scavenging still works better then encircling, and its now possible to encircle enemy snakes without too much trouble. You are still mobile enough to avoid getting targeted and killed if you watch out.
You are at high risk of being encircled now, so run if you see a large snake sprinting nearby if it is curved towards you.
10k-20k size:
Move out from the center a bit. You ideally want to have a pretty low concentration of enemies, enough that you can encircle one or two fairly safely, and so there aren't tons of snakes running around everywhere.
Running costs almost no energy, it can increase the risk of running into someone, but done early also decreases the chances of someone targeting you.
Watch out for smaller snakes, as they will often recklessly target you. If you see one that *might* target you, then simply avoid them by walking back onto your own head.
Its also notable that you can encircle food from dead enemies instead of eating them directly which allows you to A) bait smaller enemies in (at which point you encircle and eat them), B) Eat them at your leisure, and C) avoid running into any other snakes that are running up the trail in the opposite direction (as at this size it may not be possible to turn fast enough).
Watch out if you are encircling someone, as you are the most vulnerable to being encircled yourself at that point.
20k+:
Your turn rate is now pretty horrible. As such you want to be in a area with a fairly low snake concentration (which is easier said then done, as you still want to be able to find decent sized snakes to eat).
Be very careful scavenging, encircling food is now often the better choice as it makes it unlikely for enemies to sprint forwards at you.
Sprinting makes your turn rate even worse, but it also makes enemies less likely to target you. Sprinting costs almost no (proportional) energy at this stage, so don't worry about the energy cost at all.
You should spend a lot of time walking over your own body until you see a chance to encircle someone or a free path. Don't go down canyons between other snakes even if it seems safe unless you are sure that there is enough space to turn around.
If there is anyone within a few seconds of your head, simply nope out (either by turning across yourself, turning back onto yourself, or sprinting forwards). Even if they probably won't target you, a sprint can kill you due to your terrible turn rate. Being cautious does greatly slow your growth rate, but makes up for it in the not-dying area.
I can't stress the previous two points enough. What was safe at 10k is no longer safe at 20k (and is simply reckless at 40k), and will get you killed due to your horrible turn rate. Simply avoid getting in situations where enemies can kill you if they act cleverly by being super safe.
Use the noose trick when encircling both to kill enemy snakes at the end and to ensure that a smaller snake doesn't manage to encircle you.
Make wide semi loops passively so that if you see an enemy you might be able to encircle you are in a good position to do so. It helps that you are now so large that enemies sometimes can't even see you encircling them until its too late. If you manage to make it past 30k you are often so big that other snakes will almost always underestimate your size, allowing you to do a huge screen wide circle around them sprinting the whole time (and they won't try to escape since they won't believe that you can get all the way around them).