Guys
I've been playing with settings from the first post and other things these days and so far my conclusions are:
Right after reboot the phone is fast and smooth, but in time performance degrades severely up to the point it becomes unresponsive for several seconds. Right now I am using only swappines 30 and nr_requests of 128 and the overall performance is great. Reducing nr_reqauests to 4 reduces io throuput to several K per second so it is not " option for me.

To test performance I was extracting ubuntu image on EMMC. While doing that I was changing different settings of io scheduler, including changing scheduler to to noop. With moderate result. My point was - why when one is copying file over USB there is absolutely no lag, but when doing same thing on n900 it freezes. The last thing to do was to play with swap - bingo, at the moment I swapoff EMMC swap, extracting speed raise to about 10 MB/s with no lagging at all. And the strange thing is that performance raise at the moment I did swapoff, even before swap was transferred to sdcard swap partition. Swap priorities do not matter - as long as EMMC swap is turned on, performance degrades severely. And I cannot see relation with behaviour under USB.Will continue to play with that and will report if find something