It's written OK, but it sounds unnatural. "Could you speak more slowly, please?" would be a better alternative.

You could say.....

Could you slow down slightly please.

You don't have to include the verb 'speak'.

Do you think you could slow down please?

Please slow down. I don't understand.

Can you speak slowly? 

There are many ways you could say it. :)

Sincerely,

Tutor Stephy