QGIS 3.8 brings a highly-desired fix to layer effects' blurring radius (aka strength). In earlier QGIS versions, these blur settings did not take into account the DPI of the map render — resulting in inaccurate effects rendering when exporting canvases/layouts at high resolutions. In 3.8, this was fixed and high-resolution map exports now match the results shown on the map canvas and are completely resolution-independent.
This feature was developed by Mathieu Pellerin