You MUST lube the blade continuously during a cut, especially on the fairly big stuff that you are doing. I keep a squirt can of 30W near my saw and keep the blade dripping the whole cut. Your description of slow cut rate and blade drift are classic signs of a dull blade. Change it, then lube it.
I have always used HF blades and get reasonable run time out of them but nothing like Beenaway is getting with the name brand ones. I guess I'll try a Norton next time to see if it lasts better.
Point is, a sharp blade with the proper tooth pitch that is well lubed should make quick, accurate cuts for a long time.
Haven't really paid much attention to spring tension. I guess I keep it pretty light. As stated by others above, a sharp blade with aggressive enough teeth will virtually eat its way thru the steel and seem to suck itself thru the thin walls of a tube.