I think that there are many trains each day between Oxford and Paddington for which 5 carriages are
ample (I travelled on one yesterday, anectodally!

), there's definitely no need for all those services to be longer (though of course there is for some, especially, in my experience, at weekends!) ...
Separately, and to the point, it's a bigger deal than shaving a
few minutes and a
single change from the journey: Oxford to Bristol (Temple Meads) is often (usually?) two changes, at Didcot Parkway and Swindon, with the consequent delay risk and general inconvenience (and sometimes a single change at Reading, which makes for a more expensive ticket), and the time reduction is from ~1h31m to 1h9m. That's a
big difference, reducing the journey time by about a quarter (24%). The driving time for the same route (station forecourt to station forecourt) is 1h57m, so the train easily beats that either way, but for an example door-to-door journey I take between locations in the two cities, the fastest public transport route is 2h31m (a bus and four trains, as it happens!), whereas by car it's 1h44m ... getting closer to getting me out of the car, but not there yet; reducing station to station times by such a percentage (and a higher percentage still for Swindon / Chippenham / Bath Spa, of course) is a really important improvement to the service!
Absolutely agree!
(Summary of times and prices from Oxford to Bristol Temple Meads on Saturday morning from 11:00, prices at the time of writing):
Oxford | Bristol | Changes | Duration | Price (trainsplit) |
---|
11:04 | 12:35 | 1 (DID) | 91min | £29.50 |
11:17 | 13:00 | 1 (RDG) | 103min | £39.70 |
11:32 | 13:00 | 2 (DID, SWI) | 88m | £29.50 |
11:32 | 13:28 | 3 (DID, SWI, BPW) | 116min | £26.19 |
12:03 | 13:12 | 0 | 69min | £29.50 |
12:06 | 13:36 | 1 (DID) | 90min | £29.50 |
12:06 | 13:59 | 2 (DID, BPW) | 93min | £26.19 |