Yeah even though overall CPU is at 35% it looks like CPU3 core is busy at 100%. My guess is it's exporting a big image or multiple images on that core. Even though the program is multi-core it might still use just one core for the heavy lifting in the export process. If this is the bottleneck then a better CPU would speed things up, of course along with an SSD to write the files out quicker. Best bet is to get a new computer with a faster CPU and an SSD drive imo.