A lot of the time these landing sites / buildings are a bit further inland. At the beach there is usually only a buried armoured cable leading to a concrete slab or something where it connects to underground ductwork.
If you're interested in that and have an hour of time there is a youtube video[1] of a local tracking these down around the Cornwall area.
In my experience this is partially true (e.g. Carriers reusing one of the DC-HSPA Carriers as a Carrier for LTE or 5G in Band 1 is pretty common around here in Germany).
I suspect the biggest reason for the speed decrease is increased usage though.
Back when 2G/EDGE was current technology I got 100-200kbps there most of the time. Today it is basically unusable as you get timeouts constantly.
IIRC an EDGE Cell could deliver the full data rate to about 8 stations within the coverage area or thereabouts at the same time.
This worked okay back then when there were only a bunch of nerds per cell tower using but it can't cope with current usage density and patterns at all.
At least around here (germany had famously high rates for data traffic at the time when 3g was recent) smartphones and data plans are a lot more widespread than a few years ago and usage patterns got more data-heavy as well.
If you're interested in that and have an hour of time there is a youtube video[1] of a local tracking these down around the Cornwall area.
[1]https://www.youtube.com/watch?v=K_nnUbX7uuQ