Yeah - fit. I raced a Lambert for my first season of racing. Went faster than I had ever gone before on a regular basis. Next year I got serious. Worked in a bikeshop and rode most mornings on the famous "Allis Loop" of the Boston racing crowd. Tracked my times. (Long, long before power meters, heart rate monitors, etc.) That May the shop mechanic told me I should buy the last year's Fuji Professional in the basement. I did. Set it up with most settings roughly mid-range. Went for an easy "Loop", easy because it was mid-season and these cranks were 175, not the 170s I'd always ridden. Going easy, never breaking a sweat I matched my best Loop time ever! Three days later 2 minutes came off that. The rest of that season I flew like I never dreamed was possible for this body.
Now, everything on this bike was different from my previous bike except the wheels, gear ratios and seat. Position was quite different. I was both lower and more stretched out. In hindsight, I know that my breathing was far better because my chest/torso was stretched, not compressed. These are order of magnitude difference, not subtle tweaks. Like putting a compressor or supercharger on your stock engine.
Document this bike. The key fit dimensions. Height from bottom bracket to seat. Distance back, BB to seat. Same for BB to handlebars. Don't by a bike that you cannot match those dimensions. 45 years later, I ride bikes set up to fit like that Fuji. (Yes, I've changed and the position has evolved but it isn't fundamentally different.)