The time to generate a playblast depends on the intermediate (temp) file format used and there is trade-off between temporary file size and playblast speed.
Starting with v1.3.1 this is a user configurable option on the Settings page:
movie
Fastest playblast time, largest temporary file size
Note: If the temporary file size exceeds 2GB a different format will be required
png
Slowest playblast time, smallest temporary file size (default)
tga
Between movie and png in terms of playblast speed and temporary file size
Note: Temporary files should be removed automatically after the final output has been encoded.