There’s a bug in the Launcher version of the Editor that causes UHT to always run, causing iterative development to be slow.
After some digging around in UBT, it seems like it is caused by the following files being newer than the ReceiptFile:
Engine\Plugins\ScriptPlugin\Binaries\Win64\UnrealHeaderTool.modules
Engine\Plugins\ScriptPlugin\Binaries\Win64\UnrealHeaderTool-ScriptGeneratorPlugin.dll
Engine\Plugins\ScriptPlugin\Binaries\Win64\UnrealHeaderTool-ScriptGeneratorPlugin.pdb
EDIT: Quick fix is just to resave Engine\Binaries\Win64\UnrealHeaderTool.target (the ReceiptFile)