I've done this route two ways - going through the balkans ona750 elefant took a lot longer, but I wasn't pushed for time so could have gone quicker. The route is more winding and there are a lot more borders to stop at than the alternative.
In October I did UK to Istanbul in 4 days via Italy on a 45 year old bike. 1 day to Dijon, one to Modena (via Tunel de Frejus, cold round there, will be worse now), then one day down to Bari, ferry to Igoumenitsa overnight then 1 day to Istanbul. Sore arse but admitedly uncomfortable seat and mega vibrations.