Setting rate limits

Hi all,
I’ve set up a simple hotspot at home. I’m experimenting with rate limits for a fallback user profile, that the users will get once they’ve consumed their allowed quota.
So I tried setting rx/tx rate limits like 56k/56k (in the hotspot user profile) thinking I’d resurrect the feeling of using an old modem.
Testing this user profile, I’ve got mixed results. Some web sites behave as I expected (an online newspaper site, for instance, take ages to load all the photos and fancy fonts, and the limit seems respected as far as I can tell from my laptop network monitoring tool). I also tried even worse like 12k/16k and the Mikrotik webfig took so long to load that I thought for a moment it was dead. But HD Youtube videos play fine: they’re happily downloaded at > 300 kB/s, as if I had set no limits at all.
Any idea of what I may have missed?

Thanks,
P.