I noticed a difference going from 25 Mbps to 50 Mbps only when using multiple devices, and it was minor.
But the jump from 50 Mbps to 75 Mbps wasn't noticeably different at all even when using multiple devices maxing out my bandwidth.
Yep, people really over-estimate what they need, and stuck in thinking (as OP mentioned) "more is better", rather than understanding when "enough is enough" and "more is just more". We need some numbers!
https://www.vdocipher.com/blog/video-bandwidth-explanation
Streaming High Definition (HD) Video 5 – 8
Streaming Ultra HD 4K Video 25
So even three simultaneous Hi-Def streams would need < 25 Mbps, but as noted, you probably need a higher rated speed to assure you get that when the system is loaded. I'm on a 75 Mbps service, DW and I often watching hi-def videos, and buffering is super-rare, most likely a temporary slow down from the service provider, rather than the internet itself.
edit add: I ran speedtest, and I'm 'only' getting 30 Mbps at my laptop on my 75 Mbps plan, which could be any number of things either the system is slow, my modem may need to be rebooted, and/or my wifi connection is a bit week, reporting 44% strength now. But even at that, multiple hi-def youtube videos play just fine. Maybe I'll try loading them up to see where is starts slowing down - I might hit the limit of my laptop graphics before that?
Update: OK, ran 3 HD full screen streams, audio kept coming through on all, I had them full screen on separate "virtual desktops" on my Linux system (15" laptop with external 'extended' 24" monitor). So I could obnly watch one at a time, but no buffering noticed. I did sometimes get a little lag when switching to that workspace, but I think that was just my computer catching up to fill the display - plus I have tons of windows open, have not rebooted in 11 days, and am using swap memory, so none of that helps). I don't know if youtube adjusts the data rate if you have a smaller window open for the video, but I assume not, they send what they send.
Then I ran the 3 videos in a single window so I could watch and listen - no buffering, and you can see the gray area in the timeline always stays ahead of the video. My process monitor showed only about 10 Mbps at that time, and a simultaneous speedtest still gave 30 Mbps (again, a supposedly 75 Mbps service).
I can definitely tell the difference. I have 445 mbps this morning and I'm glad I don't still have the ~200 mbps service I used to get. ....
I'm curious where you see the difference. I wonder if it could have been to maybe some defective modem that got replaced when you upgraded?
-ERD50