The limiting growth factor in animal feed is protein.
To what extend protein is actually useful, is determined by its quality; protein quality.
Animals (and humans) need the essential amino acids that protein is comprised of. For poultry, the limiting amino acids (that determine the usefulness of protein) are lysine, methionine and threonine, usually in that order. If one of these amino acids is lacking, that protein is useless for growth. Exactly how useful protein is, depends on its amino acid profile, compared to poultry amino acid requirements.

Poultry amino acid requirement ratios
(0-3 weeks old) Lysine 1.1 - Methionine 0.5 - threonine 0.8
(3-6 weeks old) Lysine 1.0 - Methionine 0.38 - threonine 0.78
(6-8 weeks old) Lysine 0.85 - Methionine 0.32 - threonine 0.68

Compared to the feeds below, lysine is always the limiting amino acid

1 gram of (yellow or white) maize protein contains:
38 mg Lysine
25 mg Methionine
51 mg Threonine

1 gram of Spirulina protein contains:
53 mg Lysine
20 mg Methionine
51 mg Threonine

1 gram of wheat protein contains:
27 mg Lysine
17 mg Methionine
28 mg Threonine

As a result, Spirulina protein quality is best; 39% better than maize protein (53/38) and 96% (53/27) better than whole wheat protein.
This means that 100 grams of Spirulina protein has about the same effect on (poultry) growth as 139 grams of maize protein, or 196 grams of whole wheat protein.
How much protein is contained by Spirulina, largely depends on to what extend it is still wet, or dehydrated (eg due to sunlight exposure). Fully dehydrated / dried, Spirulina may contain up to 65% protein. Fresh, completely wet spirulina may contain only 6% protein, as then most of it is made up of water.



Sources
FAO; Lysine and other amino acids for feed: production and contribution to protein utilization in animal feeding
Evaluation of Lysine, Methionine and Threonine Needs of Broilers Three to Six Week of Age under Moderate Temperature Stress
Metabolism and Nutrition: Amino Acids - Poultry Science Association
Nutrient Requirements of Chickens - The National Academies Press
USDA Nutrient Database on wheat (click on "Full Report")
USDA Nutrient Database on corn (click on "Full Report")
USDA Nutrient Database on Spirulina (click on "Full Report")