I have had issues with EndNote running slow (screen refreshing, hanging etc) for a number of years (despite having fast PC) and have contacted Support several times and tried different things as suggested without luck.
Read somewhere that things like Smart Groups etc slows things down considerably and could be the cause. I deleted any Smart Groups etc and now EndNote runs as it should. This clearly signals a problem with the software per se - what is the point of having features like Smart Groups if they degrade performance?
Also, another gripe, what is the point of having multiple libraries if you cant search across them at the one time? In Reference Manager for example, this was possible as was the ability to cite from multiple libraries into one document without fear of crashing.
Perhaps EndNote needs to be rebuilt with some user insights…