My Nexus 6 seems to be lagging a lot since I installed Marshmallow about 2 weeks ago. Especially obvious after waking from Doze (perhaps to be expected) but also long after.
1) I actually side-loaded Marshmallow, rather than wait for the OTA update. I had to unlock the phone first (since I had not done so earlier) which, of course, wiped the entire device and, after the installation was complete the phone reset itself anyway. So, I'm not sure that an additional factory reset would help much at this point. However...
2) I have a 6P on order which should be arriving within the next 10'ish days. All things considered, I think I will just suffer a bit longer and wait for the 6P to arrive.