Starting with the release of Strategy ONE (March 2024), dossiers are also known as dashboards.
Bad performance is experienced when opening a dossier for the first time. You will also see that the slow performance is due to the slow loading of many js and css files.
Strategy split the dossier main bundle and moved the editor-related code into a separate editor bundle. During initial loading, Strategy will not load the editor bundle and will start loading it on some entry points for those editors. However, a case was missed, which is during initial dossier loading. When the dossier was first loaded there was a dependency missing and this would pull in another 60+ files one by one causing the big performance hit on that first load.
This issue has been addressed in Strategy 2021 Update 7 and newer versions. Upgrade to this version to take advantage of this fix.
There is no workaround to this issue if you are on Strategy 2021 Update 6.