The only way to increase speed is to increase cadence, stroke length or both.
Increasing cadence at the expense of stroke length may make you feel like you are swimming faster but won't make you swim faster.
Step one is to improve the efficiency and length of your stroke. Step 2 is speed it up without losing the length of your stroke.
Then you will swim faster without necessarily increasing effort.
Speed is simply a product of stroke length and tempo.
I am a certified level 3.0 Total Immersion Swim Coach.