Maybe I can shed a bit of light on this one…
I switched to a high flow hotend months ago and am not going back to a Bambu one ever…
The speed buttons are great if you want to use the slomo button to have a quite night while the printer runs.
Going faster, like in speed mode, CAN work just fine but it highly depends on the model and your expectations.
Even worse for ludicrous mode…
From what I observed it goes like this >
Sport mode comes with a slightly higher overall print and movement speed but also with increased acceleration values.
Combined it results in certain things going too fast for the desired outcome - like sharp corners, fine details, overhangs…
If you calibrated your print settings for the best possible speed AND quality in normal mode than hitting those speed buttons is a hit and miss thing.
A simple cylinder might not be a problem at all but something like a Benchy will.
Comes down to BALANCE 
As a really bad example take a 12 pointed star.
Make it bed size and it will print perfectly in normal mode.
Switch to ludicrous and the printer sounds like it tries to rattle itself into piece - not good…
So what about this badly explained max flow rate then ?
If you have a high flow hotend you quickly realise that the default speeds are RELATIVE.
I print my PLA with a max flow rate between 36 and 40.
Walls set to 200mm/s then print at around 240 to 260mm/s - no change in the settings, just a higher max flow rate.
It nicely shows that Bambu uses the max flow rate and some other factors basically as limiters.
Which explains why so many users run into issues once they start to mess with the speed settings AND the max flow rates…
Can’t speak for the H2D but for the other Bambu printers the original hotend is the main limiter and problem…
Ok, how does all this translate to normal hotend, be it the standard or hardened one?
Do the entire filament calibration first, temp, flow ratio, k-factor…
Do some meaningful test prints with the DEFAULT print profile or at least one where you did not change speed or acceleration values.
Closely check for the quality of vital things like holes in walls, overhangs, how a wall changes once the bottom is printed, top surface quality, …
Wherever you spot issues change ONE THING at a time, not tow, not three.
Like fixing the overhangs by setting the fan speeds, overhang speeds and such until you get the best outcome before moving on to fixing the bridging setting and so forth.
If in doubt create just one complete print profile first that gives you the best result for the worst possible models.
It will be a rather slow print experience…
Save this profile with a meaningful name, e.g.: PLA Slow Universal.
You now have a print profile you KNOW that it will give you the best possible quality.
Next step is to check by how much you drive certain things up.
As a rule of thumb> Start with the inner and outer walls as they are often what defines the final print surface quality.
Then do the COMBINED infill - I do this because here the flow rate is maximised and a LIMITING factor.
Crank the speed up until the infill starts to show signs of struggle, then wind it back by around 20% to be on the safe side.
If you know what you are doing you can also play with the acceleration values a bit.
Sometimes it is actually better to lower them rather then to increase them.
Why?
Simple answer> On long straights the print speed will always reach the max but for much shorter ones it does not always make sense to try a bang-bang approach.
You don’t race from one green light to the next red one if it is just 150m down the road…
It just gives a smoother overall experience for the machine if it does not have to go berserk all the time for no real reason.
You will find that a good balance between speed and acceleration provides faster prints than just trying to max it all out
Hey! You did not do anything with the max flow rate!!!
I know… But first we need to make sure the machine works as good as possible
You know have speed values that produce good prints, you fixed the overhangs, bridging and are a happy camper.
But you are going slow…
The nominal average max flow rate of 19 or 20 works fine for the simple Bambu hotends and even for PLA I doubt they would make it past the 30 mark.
Mine maxed out at just 24 ROFL
If you NOW do a max flow rate test print you not only start well calibrated, you also know that your seams or cornering speeds won’t make a mess
Means, once the max flow test starts to fail you know for sure you reached what the hotend is capable of.
Since there is always difference between batches and reading the visual results can be tricky it is best to go a bit below the max the test stated.
Let’s say from one speed step to the next the print started to fail.
That means the PREVIOUS layers were already maxed out and that you should go with the value for the layers BELOW that one
Once set to higher value do a test print and see how much faster it prints without having to change any settings.
It can also mean that some speeds are now too high !
The benchy is a good judge, so use it
Fear not though with speeds going over board as it will be limited and easy to fix, more often than not it is the outer wall speed only…