Poor performance on RoundCube using ISPConfig 3

[todx]

I've been using ISPConfig 3 for a year or so. I've got a bunch of email accounts setup, each one has 5000+ emails in it. Whenever I go to RoundCube and click on Inbox it can take up to 2-3 seconds for the list of emails to load (locally not over the internet). This server is running on high-end components. Also not on SCSI but SSD - so disk and internet load shouldn't be a problem.

Now my question. Can this be result of poor server configuration? Or is there a problem with RoundCube? What should I do? Am I using the wrong email client?