We give a uniform lower bound for the polynomial complexity of all Reeb flows on the spherization (S * M, ξ) over a closed manifold. Our measure for the dynamical complexity of Reeb flows is slow volume growth, a polynomial version of topological entropy, and our uniform bound is in terms of the polynomial growth of the homology of the based loops space of M . As an application, we extend the Bott-Samelson theorem from geodesic flows to Reeb flows: If (S * M, ξ) admits a periodic Reeb flow, or, more generally, if there exists a positive Legendrian loop of a fibre S * q M , then M is a circle or the fundamental group of M is finite and the integral cohomology ring of the universal cover of M is the one of a compact rank one symmetric space.Proof of (i). We will see in the proof of (ii) that if π 1 (M) has subexponential growth, then M belongs to the list in (ii), and π 1 (M) has polynomial growth of order 0, 1, 3, or 4.Proof of (ii). A main ingredient of the proof is the following Lemma 3.3. Consider a closed orientable 3-manifold M. If π 1 (M) has subexponential growth, then M admits a geometric structure modeled on one of the four geometries Ë 3 , Ë 2 × Ê, 3 , Nil .Proof. The proof can be extracted from [5], and is repeated here for the readers convenience. We distinguish several cases. Case 1: M is not prime. This means that M can be written as a connected sum M = M 1 #M 2 with both π 1 (M 1 ) and π 1 (M 2 ) non-trivial. By the Seifert-Van Kampen Theorem, π 1 (M) is the free product π 1 (M 1 ) * π 1 (M 2 ). It follows from the existence of normal forms for free products that π 1 (M 1 ) * π 1 (M 2 ) contains a free subgroup of rank 2 unless π 1 (M 1 ) = π 1 (M 2 ) = 2 , see Exercise-with-hints 19 in Sec. 4.1 of [56]. Our hypothesis on π 1 (M) thus implies π 1 (M 1 ) = π 1 (M 2 ) = 2 , and so M = ÊP 3 # ÊP 3 . This manifold has a geometric structure modeled on the geometry Ë 2 × Ê, see [79, p. 457].Case 2: M is prime, but not irreducible. Then M = S 2 × S 1 , see [42, Proposition 1.4] or [45, Lemma 3.13]. In particular, M has a geometric structure modeled on Ë 2 × Ê.Case 3: M is irreducible. We distinguish two subcases: