You're very close. The only suggestion is spelling out the number and dropping "with".

Past tense: The only song he could play was "Twinkle Twinkle Little Star" ten times slower than normal speed Present tense: The only song he can play is "Twinkle Twinkle Little Star" ten times slower than normal speed

"The only song he could play is twinkle twinkle little star with 10 times slower than the original speed" It needs a little reworking. "with" is incorrect usage above. I'd say, "The only song he could play is twinkle twinkle little star, but 10 times slower than the original speed", or "The only song he could play is twinkle twinkle little star but at 10 times slower than the original speed"