Warp stabilising a nested is very slow

I've been nesting clips that I need to Warp Stabilize which have change the clip speed for many years. But in the last two versions of Prem this have become a very slow process. Taking some 4 sec clips about 2 minutes for the analyzing and adding the effect to work. 

For example 

A reversed clip (-100% speed) of 4 secs after nested the effect takes 2 minutes to apply

BUT

the same clip at it orignal speed (100% speed) and NOT nested the effect takes about 10 secs to apply

BUT

in older versions of Prem the nested version may only take the same of a very little more.

The hardware I use has not changed the only thing that has is Prem.

Has anyone has the same problem and does anyone know how to speed up the apply time.