There are more tradeoffs involved in building a browser than just "powerful extensions vs browser performance". So you have a false assumption built into your question, that the two browsers in question are making identical compromises across all facets of things when that is in fact not the case.

And there are more than two browsers - if you do want a browser that still supports powerful extensions at the cost of browser performance, I hear Pale Moon is a good option (I haven't used it myself, so can't comment).