I don't think what you are saying is true for me. I have an iMac 2009 with an SSD on High Sierra and use it multiple hours every day. Video playback is great, office use is also great. It is in general snappy and I seldom experience any performance problems with it, not even in Teams calls or while multitasking. It has an Intel Core 2 Duo 3.06 Ghz and 8GB RAM. I ran Geekbench and it scores 410/785. That would, in your categories, be between "Too slow" and "More tolerable". But I would say it is for everything except gaming and video editing at least "Decent". In comparision to other computers I have, it does of course lag behind newer ones, but the SSD really does not make it feel that old.
I am wondering on what basis did you list these numbers and your comments, or what are your standards considering performance?
And another question: I know that 1000 should represent an i3-8100, but is this for single-core or multi-core, and which one should (single-core or multi-core) I look at if I'm looking to judge the performance?
Well, YMMV, but IMO 400/1000 is kinda slow. Is it usable? Sure, but it's still kinda slow.
If you want to know what I'm talking about, try visiting a few multimedia heavy web sites without an ad-blocker. Depending upon the content, it can sometimes slow right down, especially if you want to try to play an embedded video at the same time. Not always, but sometimes. Am I asking for too much? I don't think so for 2022, since this type of surfing (without ads turned off) is extremely common for mainstream users. Why not use an ad blocker? I use ad blockers for my older machines, but I sometimes run into compatibility issues and many people prefer running without ad blockers for this reason.
I'm probably biased since I have much faster machines at my disposal. Still, that's why I don't generally recommend machines that slow for novice buyers, unless budget is the absolute primary concern.
OTOH, at double those speeds, while they still aren't very good for video editing and such, the speed improvement workarounds are much less necessary and thus the machines are much more pleasant to use IMO. Furthermore, machines around my "Decent" category are generally relatively inexpensive in 2022, which makes it a reasonable starting point for new buyers of used Mac hardware. As for the 3.06 GHz Core 2 Duo, you'll get differing opinions in this forum about that. Some like you consider it more than adequate but others consider it way too slow. I'm not in the latter category but it seems I lean more toward the latter than the former. However, it seems I'm just one notch more critical of the speed than you are, so ultimately one might even argue we're almost in the same ballpark, and this is a subjective assessment after all.
BTW, as mentioned, my daughter was OK using the 2.26 GHz Core 2 Duo MacBook Pro (8 GB RAM, SSD) for some stuff, but she simply couldn't use it for some other stuff - online educational games - even though those simple games are just aimed at 9-10 year olds for school. These games are quite low impact and run fine on crappy Chromebooks but still would bring that 2009 MacBook Pro to a crawl. I've since done more due diligence researching pricing of old models and ended up getting her a 2015 MacBook Pro with Core i5-5257U - Geekbench 800/1750 - and now there are no such slowdowns anymore. I was previously considering a 2012-2014 MacBook Pro for her, but one of those would have only saved about $75-$100 US vs the 2015 model and wouldn't have been able to run the latest OS. Not worth the small savings IMO, at least for us.