Vegetarian swimmers in Egypt are routinely given mycoprotein as a way of building muscle and as a substitute for animal protein. Is mycoprotein known to improve swimming performance and muscle development?
Mycoprotein is a plant-based protein that carries a strong/full amino acid profile. It is supposed to be better for you because there is no cholesterol and is rich in vitamins and minerals.
Your question, however, is will this specific kind of protein improve performance in swimming or muscle development?
In short, yes, but in general no, and let me explain.
Yes, protein supplementation should assist in muscle development but probably more so on the recovery end, especially as a vegetarian. However, would it have any more benefit over any other soy based protein such as Spirutein or Hemp Protein? Probably not. Protein supplementation is to hit macro nutrient ratios based on your training regimen, body-type, and goals.
Will it improve swimming performance? It's possible, but it's also possible several other supplements including supplements with other types of proteins will improve swimming performance. If you're a vegetarian, protein is tough to get, however, I think in general you would need a more profound source of carbohydrates to boost your performance in the pool.
Lean protein such as Mycoprotein is a great addition to your diet, but it won't directly affect your swimming performance unless you're training properly and building muscle. The combination of Protein supplementation (in general) plus a well balanced diet, and energy source - along with your training will be your best bet and getting better.
Hope this helps.