Why unprotected cruisers were often too slow to escape

Could unprotected cruisers rely on speed for commerce raiding and survival?

In theory, but many were built cheaply with weak machinery, leaving them slow as well as unprotected.

A fast raider could avoid stronger opponents and disrupt trade, but speed cost money in machinery, boilers, and fuel. Navies that economized on armour frequently economized on the propulsion plant as well. The resulting cruiser could neither absorb punishment nor reliably escape a stronger enemy. Its low speed therefore compounded its lack of protection instead of compensating for it. Many ships did not receive every theoretical advantage associated with their design category.