Reactions generally require (more or less) two reacting molecules to collide with each other. In the case where individual reaction events are fast, then the reaction rate essentially becomes diffusion limited. That is, the rate of reaction is based on how fast two reactants can move about through the reaction medium (whatever that is). Another way to put this is that a reaction cannot go faster than the rate at which reactants travel through the reaction mixture. Solid-solid reactions tend to be slow not because reaction events are thermodynamically unfavorable, but rather because molecules in solids are practically immobile. So, effectively only interfacial molecules can interact - and only when the interfaces are in direct contact, which even then is infrequent because of insulating air spaces. When two solids are dissolved in a solvent, two things happen: the effective surface area of the reactant is reduced to basically the molecular size (every molecule is at "the interface") and, probably more important, the diffusion rates of reactants increase substantially, which dramatically increases the probability of a collision between reactants. In a word, solvents facilitate fast and homogeneous mixing. As discodermolide has mentioned, you usually want a concentrated solution. Concentrated and dilute solutions have (to a first approximation) the same diffusion rates of reactants - the difference as far as reaction rates goes now becomes how far on average a reactant has to diffuse before a collision takes place. This is also why reactions slow down as they proceed - the reaction concentration decreases as reactants are consumed, which increases the time it takes for remaining reactants to find each other by diffusion through the solution medium.