That system really should be plenty powerful for that amount of tables.
Could you please open PT3 with the logging enabled (there's a link from the start menu with logging enabled) and play until you experience this lag, and then submit a bug report? That will help us look into what specifically is causing your lag.
I know there are some performance improvements coming in the pipeline, though I can't say for certain how effective they are yet as they aren't available in alpha testing (hopefully they will be soon, though).