I am running a middle relay on a raspberry pi b+ its been up for about two weeks now and until last night was averaging 50MB downloads a day. Now its finally sped up, hope im not doing something wrong my relay has HSDIR tag on it not sure what that means.
You're not providing enough information to answer the question but you can always take a look at the status of your relay at Globe or Atlas. Familiarize yourself with what each of the flags mean, that will help you diagnose if there are issues. HSDir means that you are hosting a portion of the hidden service distributed hash table or the Hidden Service Directory.
You can also make manual circuits to confirm that you can indeed use your relay as a node. This is pretty easy if you're using STEM.