Putting “root on SSD and home on HDD” in a search engine will yield countless results with the same conclusion: /home on HDD will slow down your system.
Example search result:
https://askubuntu.com/questions/1385455/is-keeping-home-in-hdd-and-root-in-ssd-impact-performance-or-keep-both-to-ssd
There’s really no way for anyone to know what’s going to be optimal for you. Nobody could have predicted that you would be concerned about a 5s slowdown in loading Firefox. Most people don’t get concerned about that. 
I would also suspect that most people might have assumed that an SSD being faster than a HDD is obvious and didn’t need to be explicitly called out, so if you were seeing a partition proposal that put some of your system on a HDD, that it would be apparent that accesses to whatever was on that partition would be slower.
Online forums are about providing the best help given the information provided, but it’s always a “best effort” by those who are involved. There are no guarantees that you’re going to get a fully optimized answer to any given question.
Please… don’t get me wrong.
I do not accuse anyone!
Actually I’m absolutely grateful for the forum’s members. Really 
But I’ve to announce there will be a thread of newbie user42 how to savely
change my kind of complex partition scheme to have /home at a separate
partition of the SSD.
Do not complain - it’s your own fault, if you provide me with knowledge!
Appreciate the clarification.
Just trying to help set expectations here, in the event that they weren’t clear. Everyone here is a volunteer and does the best they can with the information they have available at the time.
You might consider replacing the HDD with an SSD. (Heck, I ordered my current system a little over 2 years ago, and even though I placed the order knowing that the system was entirely SSD-based, for about a year, I somehow was under the misapprehension that my /home was on a HDD (and the system was on an M.2 SSD, which it is). 
Imagine my surprise when I “discovered” that it was in fact an SSD after all).
Redoing the partitions on a system is a pretty complex operation, and you might find that the drive isn’t big enough to give you enough space for /home for what you want to put on it. You might find that putting the Win7 system in a virtual machine is a better option long-term (for one thing, you can throw it behind an additional NAT layer, making it harder - but not impossible - to compromise; for another, you can create a snapshot of it - as long as critical data is not stored inside the VM - so you can revert it in the event it is compromised).
So, one of the MAIN reasons people use a different partition for /home is so that /home is not affected by a catastrophic failure of the “system related” partition(s).
But that does not mean it has to be on a separate drive. Though, that is a good idea.
On our single drive machines (like a laptop), we use a separate /home , formatted as XFS.
The system is BTRFS.
On the desktops, one SSD is partitioned and formatted as BTRFS for the system partitions … a second SSD is dedicated to /home, formatted as XFS.
We back up /home’s weekly.
We don’t worry about backups for the system.
Thank you for all that.
More & more I get better understanding of the principles and “best practices”.
@hendersj
Switching to SSD completely is no option for me… as it’s about 10+ TB
(I would have to sell the Ferrari).
Win7 (beloved, did I mention?) I will drop as soon as I can do in openSUSE
everything as good as in Win. The problem is that I’m deep in development, media work
and several other areas quite specific. I haven’t found ways to deal at this level
in openSUSE (yet). But when it is as far, I plan to install the new Leap upon everything.
(It was always a relief for me to kick off a once beloved to have tabula rasa 
)
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