Air carrier Captain reported unable to exit parallel runway after landing due to another aircraft blocking a taxiway resulting in a runway incursion and go-around. Captain indicated airborne traffic on final while their aircraft was blocked from exiting the runway onto the taxiway by a conflicting aircraft taxiing very slowly and not expediting. ATC instructed the aircraft on final to go-around due to the aircraft remaining on the landing runway and was blocked from entering the taxiway.

Narrative

We followed an Aircraft Y on final for 28R. The Aircraft Y cleared the runway at Taxiway Tango and were instructed to hold short of 28L. They complied and were holding short of 28L when we touched down on 28R. On our roll out we heard Tower clear the Aircraft Y to cross 28L. We were cleared to cross 28L approximately as we passed Taxiway E. As the Aircraft Y was on tango blocking us from clearing 28R in a timely manner; we continued to the next taxiway and exited at delta. As I turned onto delta; I saw the Aircraft Y moving. I was across the 28L hold short and well on to 28L when I realized the Aircraft Y was moving quite slow and not accelerated to a normal crossing speed. I had to stop on 28L to avoid contact with the Aircraft Y. The Aircraft Y moved slowly across the runway; never expediting in my view. It seemed they were single engine. The aircraft on final was getting pretty low. I considered a radio call. But I'm all too familiar with the disastrous results of a blocked transmission in ZZZ1. Maybe it was my perspective. But I thought the Tower go around call was a bit late. Things were looked tight enough I started to develop alternate plans to exit the runway if needed.