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Form editor tries to run all UI modifications on edited forms in a L&F environment where only the original UI defaults are present (not that customized by the NetBeans IDE). Switching between the IDE defaults and the "default" defaults is not very optimal - it copies large UIDefaults tables and resolves all lazy values from there. This could be hopefully optimized somehow while keeping the designed forms looking reasonably well (i.e. as in an unmodified default look and feel).
Look and feel swapping was rewritten several times since this issue had been created. The rewrites took performance aspects into account => closing as fixed.