WebDirect Slowness on Ubuntu Server

Hello,

Last week I installed FMS 19.3 on Ubuntu Server 18.04, following the instructions in the article "The Penguin is Back!" The machine is on our internal network, which has no access to the outside world.  I expected the performance to be pretty snappy with WebDirect, but that's not what I've seen so far.

When I access the admin console from another machine on our LAN, it responds and renders almost immediately.  But when I call up the WebDirect menu page, it takes about 10 seconds to load.  Right now, I only have one small file on the server.  When I click on its icon, it takes about 10 seconds for the login prompt to appear.  Then, after logging in, it takes it a little while to correctly render the first layout.  The layout is pretty simple - just a button bar and some text.  But sometimes the button bar will just be outlined without the fill color for a few seconds.

What are some things I could check for proper operation?