Change 1000+ of WordPress Featured Images in 5 Minutes! › Forums › Questions and Answers › Screen Timeouts › Reply To: Screen Timeouts
The plugin tries to set via PHP the max_execution_time to 3000 and the memory_limit to 512 MB. If the server does not allow increasing these options to those values I recommend to talk with your provider about that.
And you can narrow down the time period on a daily base to have less posts in the process.
Another bottleneck could be the connection between the external server and yours. Since mostly we have no influence to that decreasing the number of transmitted images to keep the execution time short is the way.