Replication should have a huge number of specific counters to choose from. Several of the agents have a Delivery Latency counter which will tell you if the replication is behind or not. You want that number as low as possible. The higher the number (you'll have to see what your environment produces) the further behind your subscribers are.
As for effecting performance on the server if your distributor is running on the publisher that can cause performance issues, especially if you have a lot of Subscribers that will impact the load on the publisher. If they are separate then there shouldn't be a lot of load on the publisher. If you have them on the same server, consider moving the distributor to it's own server.