That's exactly why the quality has gone down, due to these speed subs. Back in the day speed subs were not a thing. They popped up when legal subs started popping up, almost as a way to compete.
I think the first big name speed sub group was horriblesubs, and they still exist today. Today fansubs are dominated by these groups. They're quality is okay to good. Sometimes they're the only option now, so I'd say the quality has gone down.
Back in the 4:3 days when anime was vbr and sources were interlaced fansub groups would go through frame by frame doing pulldown and denoising. It was amazing. They'd have high quality karaoke in the opening and ending scenes. When's the last time you've seen high quality karaoke on fansubs?
No, quality has gone down. They don't even do color correction today. Maybe in the last 3 years or so fansub quality has gone up a bit, but it's not a lot.. not enough for me to notice much.
>speaking as an amateur translator myself I always think it's good for the viewer to have alternative interpretations for more difficult-to-translate passages.
Yah. You don't often see translation notes any more either.