device has faster fileaccess times if free memory is used for filecache like in normal linux machine, So for me Drop cache makes the device slower. Of course if you like to have free ram in your freemem list, (that doesn't benefit the performance) then drop cache might be helpful.

Usually its slow after the update, because its rebuilding its indexes, and that might take quite long if there is lot of media files or emails. Few days in the worst cases.