My wifes bike has 9 speed barcons and lx RD-M570. This worked for many years fine, then it got sluggish on shifts from larger to smaller cogs, especially to the smallest cog from its neighbor, when the spring couldn't get the job done. It would get to the point that it took a very long time to make the shift to the smallest cog. The cable would be slack, the limit screw was properly set and not limiting the real derailleur. Many minutes after attempting this shift it would sometimes drop in. Cleaning and lubing the derailleur pivots helped for a while, but eventually this did not restore satisfactory performance. Note that I wouldn't expect the friction shifting mode to help this situation at all. Friction shifting would not strengthen the spring or reduce the friction it needs to overcome. Replaced with xt shadow rd-m772. no issues, but the cable routing goes a bit wide leaving the derailleur and may touch your panniers. moderate miles on the 772.
I have another bike with 9 speed barcons and xt RD-M750. no issues, but I haven't had this bike long and it has low miles.