Slowdown and lost windows when connecting/disconnecting external monitor

When I connect my external monitor, my 2017 MacBook Pro (OS 10.15) will freeze or have severe slowdown for more than a minute. When disconnecting, I often lose windows that I can't recover through any known means except reconnecting to the monitor. Is there a way to reduce the slowdown and have it properly move external windows back to the built-in display? The monitor is an older SyncMaster using a USB-C -> DVI connector.