You might see that phpMyAdmin in user-level phpMyAdmin is slow to load on servers that have several hundred databases or more.
One cause of this issue is phpMyAdmin information schema searches, so the databases are being searched through a database containing all of the databases.
You can attempt to work around the slowness with the following steps:
1. Navigate to WHM > Server Configuration > Tweak Settings
2. Click the "Software" tab
3. Make sure "Enable phpMyAdmin information schema searches" is set to "Off"
4. Click Save