Too slow and the area near the nozzle may heat up and warp, plus the print would take forever. Too fast and the extruded filament may not bond well with the layer underneath or the layer underneath may not have cooled enough and the added filament causes the layer under it to deform a little.
Speed can affect stringing.
There is usually a range of speeds for any given printer, filament type, and even sometimes the color of filament can make a difference.
What you will begin to learn about speed is when you start to have a problem, you’ll search for what causes that problem and you’ll see info recommending a slower or faster speed depending on what it is.
For any type of filament there is usually a good starting speed. You might be able to go faster, but it’s just a general speed that often works well for people.
For PETG which I use a lot you can usually start with 30 to 40 mm/s print speed and it’ll usually come out good. The first layer should be around 15 to 20 mm/s to give the liquid filament time to hit the bed on the first layer and solidify before the head moves too much.
PETG can stick to the bed well if the bed is clean (using isopropyl alcohol to remove any oils that may have gotten on the bed from touching it.). But it doesn’t like to stick to the bed if it is being pulled by the nozzle too hard from being moved too fast.
Travel speed is just the speed between two non-extrusion moves. Say the nozzle just finished laying down a long line of plastic and now it needs to move to the next starting position. You want travels to be as fast as the printer can handle without jerking the print around and the print coming loose.
Sometime when there is filament oozing out of your nozzle, like as the very start of a print job just before it does the homing sequence, grab that filament with tweezers and slowly pull it out. That is like a slow travel move… The filament is dragged out of the nozzle and creates a long thin string. Then another time grab the oozing filament with tweezers and pull really fast. You’ll see the filament is much more likely to break off cleanly. You want fast travel moves to save total print time and it is one factor that helps reduce stringing.
YouTube channels like CNC Kitchen, CHEP, Thomas Sandladerer, and others demonstrate the effect of many print settings and help you see what to look for. I suggest you check them out and others that get recommended to you by YouTube as you start to watch those videos and “like” them with the thumbs up. YouTube will start to recommend more videos like that.
A sign of moves that are too fast is when you get layer shifts in the print (Google search that term if you are unsure what that means: 3d printing layer shift example) or if the print comes loose, or even if you see distorting in the walls. A belt driven printer can handle moves in the 125 to 200 mm/s range. A screw driven printer might be doing good to move at 75 or 80mm/s. (That’s just an approximate starting point. My V1 Snapmaker would start to grind at the stepper motor and loose steps as in the motor would skip turns and the print head wouldn’t move as far as it should at much over 75mm/s. )
Stringing that I mentioned earlier are also affected by retraction, print speed, travel speed, fan speed,and print temperature. So these settings are all additive. If you have really bad stringing you could change any one setting and see if it gets better or worse, or as you gain experience you’ll eventually just know that to reduce stringing you may lower the print temperature a little as long as the filament still comes out and still bonds well to the layer below it, you might raise travel speed a little, you might increase retraction length or retraction speed. You also may see some videos on how to print a small test object that creates the worst case scenario for stringing and you could learn how to change the temperature in the gcode file every 1cm of height (10mm of height / your chosen layer height, rounded to a whole number is the number of layers you skip in the gcode before adding a command that changes the print temperature.). So you can see after a print job what effect print temperate has on stringing. My explanation here is meant to introduce the topic to you. If you want to try it sometime, search Google. Search YouTube. There are tons of examples OR you may come across something about this and now you have an introduction to the concept dm so the next time you see anything about it you’ll have a small head start.
3D Printing is an education. It is like a class that you take yourself, at your own pace, and what you get out of it really depends on what you are willing to read, watch, and learn. There is always a chance to learn something new, experiment, and grow. The number one most frustrating thing I see are beginners the assumed the programs and the printers were just going to do everything for them. 3d printing isn’t hard, but there is a lot of details and a lot of things that you can do (once you know how) to change how your print comes out, how much plastic is used, how much time, and the finish quality.). So, if you are willing to read / watch videos, try, fail, make a change, try again, and learn, you’ll do fantastic at this! If you are not willing to learn and grow, maybe take up the long lost art of watching grass grow, I don’t know. (No, I don’t mean you personally, I’m just making a statement about commitment here… The fact that you are here asking questions and reading is already a sign that you are on a good path, just when in doubt read more, find more YouTube videos, be patient with yourself because you are learning!)
and testing, testing, playing.