These frames tend to be more flexible than frames made from metal. The range of stiffness in these frames depends on the grade of materials used. Higher quality fitness skate frames come with crossmembers to increase stiffness and performance. Look for the response system frame on the K2 Camano and K2 Velocity. Usually, the stiffer the frame, the better it transmits power.
More power means more speed.
Extruded aluminum frames are laterally stiffer than stamped aluminum frames, so transfer of energy between the skates and the pavement is more direct. There are also some other frame hybrids out there that are pushing the metallurgic envelope. The kind of skating you want to do determines the length of frame you should look for on a pair of skates. Short frames are better for quick starts and fast, agile turns. A longer frame gives you better stability when going over obstacles and maintains speed better on those longer fitness rolls.