If you can get your hands on the necessary things to clean a bearing and feel comfortable doing that, I’d recommend it. Here’s a post on traditional bearing cleaning methods: How to Clean Your YoYo Bearing. And here’s a post on the paper bearing cleaning method, which you could do instead of or in addition to the above: On bearings and the paper cleaning method: - #15 by A_Soviet_Locust.
I don’t know if it affects the speed per se, but it can definitely change the feel of a yoyo for the worse. If you can exchange the string for a new one, then that’s definitely a good idea to try.