Redis won't have much effect on your MySQL performance. It would help if you told us...
* Moodle version
* Your server configuration (roughly)
* What have you done (if anything) to tune MySQL performance.
* How do you define "slow"
Running everything on one box can be tricky. Depending where your software came from default configurations are rarely useful for Moodle. For example, Apache will try to use all of the memory and MySQL very little of it.
You should also read Performance_recommendations