I'd suggest looking at the replication metadata retention period on the publisher. It defaults to 14 days, but if for some reason you have it as unlimited, the metadata tables start getting increasingly big and it can affect performance, publisher and subscriber. Other than that, just the standard performance things - SQL related like rebuilding indexes, using a full recovery model and having an enormous transaction log file (I see this in so many client envs where they think "oh full must be better than simple so I'll chose that", without setting up proper maintenance plans). And even physical issues like HDD fragmentation, memory.