I don’t really see what a benefit a fast service would have between Tywyn and Pwllheli. Bearing in mind the passing points are fairly evenly spaced, a fast service would surely just end up waiting at two passing loop for ages? Also, many request stops generate steady traffic to the towns on the route. Namely Penhelig, Llwyngwril, Morfa Mawddach, Talybont, Dyffryn Ardudwy, Llanbedr & Talsarnau, some of which are busier than stations such as Minffordd and Criccieth. Also, from previous experiences, the busiest part of the line seems to be Machynlleth - Barmouth/Harlech. Also, if Machynlleth are crewing a Dovey Junction starter, if the preceding Aber train (to get them to DJ) is cancelled - They’d also need to cancel the Dovey Junction - Pwllheli, or delay it whilst they send crew in a taxi which would have a knock on effect to all services on the line due to the passing points at loops.
Obviously the more expensive option but a passing hoop at Dovey Junction (or double the track between Mach and Dovey) with trains running through to at least Mach (or attaching onto ex-Aber trains) would I suspect be better for reliability, but I doubt it would happen.