I’m only ***18…*** and I had to buy a chair for the sole reason that the previous dining table chair I was using was further back than 90 degrees and I generally could never rest my back and spend 4+ hours in pugs during holidays. Instead of playing till I was mentally tired or felt mechanically fatigued, I had to play till my fucking back started hurting instead, reducing my play time, now I can easily hit 6+ hours with ease on days that I want to. So yes, it can affect how good you get by limiting your playing time.
\+ with the less stress you probably are saving some amount of energy in your muscles somewhere which should help you with focusing too.