Very slow movement on solitaire mode.

[Orelle]

We are playing Global war scenario on solitaire mode by PBEM.
Now in March-April 1943.
Starting last turn, movement of URSS and German troops is very difficult and slow due to a delayed computer response in a lot of units movement. ( sometimes it takes more than one minute to get it done for a single unit).
Game works quite fine for the rest.
Anyone knows possible reason and/or solutions for that?

Thanks in advance

Orelle