Folder browsing should be fine unless maybe the NAS is really busy, or the network connection is poor, what does your DSM Resource Monitor show?
If you have playback issues it's because the Xbox isn't capable of direct streaming all formats sent from PMS on your NAS, so it's transcoding your media into a format the Xbox can accept, which is a CPU intensive task.
Unfortunately, Synology NAS units are currently shipped with relatively weak CPU's. When the Xbox and Plex app receive further software updates (read: better codec and subtitle support) this should become less of an issue.