...That's because these are not the stopping trains, TfL» are running those..
For me, a train that calls at six stations between Reading and Paddington is a stopping one! It sure ain't a semi-fast. But I appreciate that it's not so much of a stopper as the
TFL▸ ones (11 stops).
Cue for suggestions as to what we call the slower
GWR▸ trains. (Semi-stoppers?

Marlburian
Being GWR, we can't call them "fast" and "slow"
We could have fun with this one ... can't call them "main" and "relief" - confusion with lines, wrong usage of words, and you can' relieve yourself on the relief trains.
By animal speeds ...
https://en.wikipedia.org/wiki/Fastest_animals"Falcons" - the superfasts that don't even stop at Reading
"Eagles" - first stop Reading,
IETs▸ "Swifts" - one or two stops on their way to Reading
"Swans" - about half the stations
"Ostritches" - all stations trains