All apps are limited by Shopify to 2 changes or requests per second (4 per second for Plus stores). Each field value you want to import is one change. Each field value you want to export is a request. Products with many variants will compound the problem, as each variant is another data set with its own fields.
Our app is optimized to be as efficient as possible, but we cannot speed the process up any faster than Shopify’s limits allow. It's frustrating to us too, but we have no control over Shopify's rules and limits. You can read more about this at https://help.shopify.com/en/api/getting-started/api-call-limit
To speed up exports, use the export filters to limit the amount of data you're exporting. Exports will run much faster if you're only asking for a limited set of data. You may also wish to limit your exports to only rows that include Custom Field data, i.e., a field you created in the app must have a value in order for the product or other object to be included in the export file.
To speed up imports, only import data that needs updating, rather than importing every value in your store. For example, you might start with an export file that includes every field. Remove all field columns in your file that don't need to be updated. Remember, the app will not delete data unless the field specifically has the word "DELETE" in it.