Incredible, I think I've found the culprit: ONE freaking number on the ini file. If you look on the screen of the ini file, you see there are four values for the shadow resolution. One is on the fist ini, three are on the second. Changing those values all to 2048 was one of the first things I did before posting here, but with no improvements. Well..while I was making the screenhots I realised I didn't see the THIRD value of the second .ini. Changed that too to 2048 and bam!, no more huge lag spikes after loading a cell. I still have medium to poor performance on a big and quite modded place like Whiterun, but everything loads quickly now. Just tested it. One f* number, can you believe it?