Is this Rhino functionality or smth to do with your keyboard or mouse?!
This is Rhino functionality and is built into the instant aliases feature set.
The proposal was to implement quick single-key commands in future Rhino releases by holding any of the letter and numerical key for more than half a second. Some Rhino users may prefer to activate the quick commands after a longer press, this is why the reading time must be customizable in milliseconds.
Ah, so I guess Rhino 9 exclusive?! 
Yea; this is a new feature being worked on for Rhino 9
I also want to make a similar plugin, can you tell me how you achieved it?
I have a suggestion:
Donāt activate aliases repeatedly while holding the key for a long time. This will prevent some nasty things
@tom33 - You should be able to set the delay youād like for the instant alias to kick in
Does that help?
Before releasing the key
Wonāt activate commands repeatedly?
Tried the instant for the first time just now, and Iām seeing an issue with Macs: The delay [ MacOS default] seems to activate the options selector for the lettersā¦[not sure how to it is called]
For me I guess it can be deactivate in the OS setting. But for users from places where these are needed in their languages, there may be an issue here!
Please give us the ability to assign the popup menu to a single button press. I have most of the tool icons saved to the popup menu to save time, and having to press the middle mouse buttons hundreds of times a day is very fatiguing on the clicking finger. And the shortcut binding option slows you down at least 2x.
Zbrush does this and it works so well, please take a look. This would honestly be a major improvement to the user experience.
You can create an alias for the command that opens the popup toolbar ā_PopupToolbar default.PopUpā assign it to the key you want and make the alias instant.
Thanks for the tip, amazing things are happening in Rhino 9! I just tried it, and it works great but has a few quirks.
When running the instant alias the mouse disappears for a short time, and there is a slight delay in launching the popup. Neither of these things occur when launching from the middle mouse button. I hope these issues are ironed out for the final release.
Do you mean having a shorter reaction time for the middle mouse button to trigger the pop-up menu? If so, I believe that there is an option to set that time in milliseconds somewhere in the Rhino options. I canāt check the program now as Iām out writing from my phone, but I remember that I used that option a few years ago to reduce the reaction time.
Nice catch! I found it itās under the advanced Rhino Options and called Rhino.Options.AliasList.InstantAliasDelay and setting the value to 0 from the default 300 basically fixes it. The mouse still disappears but only for like a millisecond and itās very minor.
0 should probably be the default instead of 300 
Setting this to 0 means that you cannot run any other command that starts with the letter of the instant alias.
Oh I see, thanks for the heads up. Thatās fine in my case as I would reserve that button for the popup. I have everything I use on shortcuts, aliases or popup anyway so itās rare that I type out a command.
Wait I totally missed that the delay would apply to all instant aliases, and that is actually not great at all. Can it be possible to set a separate delay only for the popup? Maybe another advanced option for popup only?
Not sure this is related to what you say, but I always asign F1 to F4 to the four views:
F1 = '_SetMaximizedViewport Top
F2 = '_SetMaximizedViewport Front etc.
Works like a charm. Even while inside a command.
F5 resets to all 4 views '_4View.
never understood why adding Ctrl as a modifier for that..
I canāt work without that setting anymore..
also ctrl - M for Move, ctr - S vor Split and other custom settings.
but I may have to upgrade my habits with these Instant Aliases! 