Target Platform .net4 to .Net8

What does switching target to Multiplaform do? It was set as .net 4.8 when it was windows then switching to mutli .net 8 there are C# libraries that are no longer found under .net 8 platform? specifically we were using System.Diagnostics.PerformanceCounter but Performance is no longer found in the namespace. NUget packages? We are having really poor HTML quality slow, not responsive, not slowing correct pages. Thought switching target to multi might clear that up.

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Target here:

Pages are set to portable

I think I have the answer the multiplatform is not the same as pages between WPF/HTML but between Windows/Linux.