So you have 8 nodes & 2 are slow. You can try the below approach to troubleshoot.
1. Are both the slow nodes are webnodes?
2. Hope PDC is installed for the system, You can verify health of both the nodes.
3.Check major alerts & exceptions for that node using pdc.
4.If PDC is not installed then download pega rules & pega alert logs. Check for a pattern of alerts & take necessary actions according to it fro example If it is Pega0005 then DB related issue, Pega0028 then memory related issue etc