Sadly its a global setting. I can’t accept this musically, for exactly the reasons you have mentioned.
Also, @Lillie_Harris suggested I could set a tempo on a run of unslashed grace notes to set a desired speed but it appears there is a bug in Dorico which messes up the following tempo and does not do the speed indicated for the graces.
I do not know the current status but I hope this has been registered as a bug in the defect list. See here:
You will find the discussion of grace note speed inside this topic, with my minimal working example that shows the issue.