[deanhills]

Thanks @Mashiro  I get it now.  Like with myVestaCP of dpeca's, his panel script must have been upgraded too - everything involved with Debian 10 and that has been written for Debian 10 must be on a higher level.

At any rate, it's been a powerful lesson to upgrade Debian 8 to 10 from the command line.  I've never done this before.  I never expected it to be as simple either.  Or with such a meaningful difference in speed and performance.

@deanhills : i have so question.
Can i use this command on my VPS-9 ?
Is this a soft upgradation command which remains all my custom programs and settings, or convert into fresh copy with zero updates.
my basic Running programs are in
/etc
/var
/usr
is this all directories still remains or all OS back to fresh?

@sagher I'd be careful if I were you as you have content on your VPS.  I did the upgrade without any content on my VPS.  Maybe we should ask @Mashiro's advice what the effect can be if you have content on your VPS.

Hello may I know when you say VPS speed increases, what kind of speed parameter(s) you are referring to?

I mean network speed, disk speed, or what else?

Normally a fresh install of an OS should run faster than the previous one you have used for some period of time.

I didn't do a technical analysis of the speed before I started the upgrade.  So I can't really compare stats other than my experience with loading VestaCP on Debian 8 and running a WordPress installation on it.  The speed was not slow when I downloaded themes and plugins and uploaded pages and posts.  But with the same installation on Debian 9 and Debian 10, the plugins uploaded noticeably faster.   In absence of real corroborating data, the speed could easily be due to different factors of course as Mashiro also suggested.