How to run pg_dump so that it doesn't slow down other connections?
Little hard to say, because we don't know your database setup. 

does your database use schemas with views in it. you can use the -n option to dump only the data schemas and rebuild you views after the restore. 

maybe you can do it on a quit moment on your server, with -j you can parallelize the dump. It will take less time, but uses more memory.

Otherwise the slave option is a good practice