5000 rows isn't laggy in Firefox on a current genetration MBP, so I've nothing to compare to. It would be good to have an additional step with 500,000 rows without clusterize (or increase the number of rows in the first step).
With this enabled scrolling is calculated in its own thread which makes Firefox insanely smooth now, so I doubt I'd notice a difference even if it were 50000. I recommend everyone gives it a try, it's a big deal.
That will be enabled in stable soon I believe.
You may test it at playground. By default there are 1000 rows, you could even increase it by hitting "Append" button, then click destroy method "Insert all rows to the table" button. After that you will be able to init plugin again. It'll allow you to compare the difference.