On my property I have several very old, very dead mesquite trees. I have trimmed them down for firewood and all I have left are the trunks. I have a hefty saw, Stihl MS661 and it is struggling to cut these. I have a 28in bar on it. I am tired of going through chains to cut these trees up. I was suggested to get a carbide (RD3) chain for it but pricey.... it is one cut, one chain. takes about 3 min to make a single cut. But i have heard the carbide chains do not cut any faster just last longer. I resharpen my chains at 30* cuts. do I need to go higher or lower on the cut angle? I am cutting across the trunk but with the hardness of the wood I dont think going in either direction is going to make a difference. its like cutting concrete. any other suggestions?