I don't know about the Arduino, but "CW Get" is free for PC's, and it will measure your sending speed. "fldigi" has similar capabilities.
For either program, you need to run your straight key through a sidetone generator (that is, it needs to have an audible "beep"). Use the computer mic to "listen" to the sidetone.
Either program will also tell you whether your straight-key sending is _readable_. What's music to your ears, may be mush to the computer.
. . . also consider moving up to a paddle keyer . . .
+1. Life is much easier with a paddle, IMHO. I know that some people disagree.