Complex sentence: Use of poppet valves also increased the speed each set of drivers could achieve because they gave very accurate steam delivery to the cylinders, however there was a drawback of the metallurgy in the poppet valve could not take the stress of sustained high speed operation.
Split into simpler sentences: Use of poppet valves also increased the speed each set of drivers could achieve because they gave very accurate steam delivery to the cylinders. However, there was a drawback of the metallurgy used, meaning that the poppet valve could not take the stress of sustained high speed operation.