Regarding speed of Maxar you can install the josm http2 plugin which should speed up the imagery load for this source. However as this a pretty new plugin you may face some bugs in JOSM.
I am wondering how this htpp2 add-ins work. I install Java 11 and http2 plugin. I confirm that the JSOM is using Java 11 by checking the about window. But I still feel that Maxar Image service is very slow as before. I am not sure whether this add-in is active.