The S905 also has this same issue. There is micro-stuttering when subtitles are displayed. I disable subtitles and it plays perfectly. It doesn't always stutter in the same spot when I replay a scene that initially had a stutter. Some scenes are more prone to stuttering than others.
I noticed that too with my 905x, but not only the first Subtitle. It appeare randomly , ca. every 20th Sub, both types, SRT in MKV or SUP embedded in m2ts/Blu-ray Structur.