I think you're misunderstanding a little in terms of how speed and google rankings relate. Although Google have never given a definitive answer (to my knowledge), the current consensus is that as long as you pass the core web vitals tests, then speed won't be a negative factor. So, once you do pass them, you're not going to gain anything in that respect by spending more time trying to get additional small gains in speed.
Out of the box, assuming it's hosted on reasonable servers which are appropriately resourced for the traffic the community receives, and assuming there are no modifications causing issues, from what I've seen at least, Invision's software passes the core web vitals tests.
So, for the vast majority of sites, that's great. If your hosting isn't great, or you have a bunch of modifications which slow things down, then you may need some extra help to pass the core web vitals test. Perhaps you were in that boat, who knows. But, personally I'd still recommend you and others in a similar position look first at the root causes of any specific problems you're having and dealing with those. Going to great lengths to minimise the css is more like a sticking plaster and a time consuming one at that.