There are some things that just don’t work at all at these speeds. Uploads are horrendous at 1.5Mbps (measured, presumably TCP payload speedtest2.aa.net.uk) - doing a backup of one iPad can take 40 mins and it just wipes every other kind of traffic out while it’s doing it, effectively putting a hold on any other activities, and has a bad effect on downloads too. Mind you a lot of the fault is down to operating systems and my router - bufferbloat and insufficient queue-, QoS and fancy traffic management features.
The other thing is iMessage which sometimes doesn’t work properly at all, with incredibly distorted picture with bizarre vertical bars on it. It’s ok on 4G though, which I view as very significant. When I’m trying to iMessage Janet in the house I think each of us is only getting 1/2 of the DSL upload bandwidth per pipe per user.
And even though I now have 10.5 Mbps downstream which is joyous compared to 49kbps/33kbps (49-200k/33-130kbps max with highly compressible data) in 2003, 0.5 Mbps downstream in 2004 and then 1.5 / 0.4 Mbps in 2006, two of us doing things at the same time is sometimes not good at all. So I avoid activities that might disrupt my wife while she is trying to work - software downloads for updates especially common case. Downloading movies is left till the middle of the night so they won’t wipe Janet out or cost a fortune (AA ‘units’ deal means downloads are 400x cheaper in small hours)