Here are a few tips to help you optimize your customizer speed if you feel that it does not load and run as quickly as you would like it to:
Reduce your customizer canvas size, the maximum recommended size is 1200 x 1200 pixels.
Resize and save your product images to match your new canvas size with photoshop.
Also make sure your thumbnail images are not too big (minimally 50px x 50px but no larger than 300px x 400px).
Compress your .png images using TinyPNG.
Re-upload your compressed images in the MyCustomizer product builder.
Smaller dimensions mean lighter weight and faster loading times!