Hi, I am facing the same issue, and I guess that it is a wide-known problem because I see many topics regarding the performance leak in all SDKs. Geoserver and WMS Tiled WMS performance with large number of layers https://community.esri.com/thread/216300-why-is-webtiledlayer-performance-horrible I've tried using OpenLayers and the performance is better and I've sniffed the traffic and it seems that OpenLayers has optimized the WMS/WMTS requests, so it works better. I haven't tried the 10.X.X release, but I guess that the reimplementation that took place in 2017 has some important performance issues. Hope this help.
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