When I bevel anything with less then 30K Vertices, it lags really bad in 2.8.
What use to take me 5 seconds to perform now takes me, sometimes, a minute to do.
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There was a change, but it was in later versions too (like 2.79) where an optimizing solver is used to get more even results when "Loop slide" is on. It could be that that is slowing things down a lot for bevels involving a large number of edges. Try turning Loop slide off and see if that improves things. You can also submit a bug to Blender tracker and I might try to see whether I can make things better.