There are a couple of things that first come to mind when you want to find why your inventory updates are running slow:
1. The timing of these updates.
Spark Shipping is limited by your site's API rate limit. Each shopping cart has a limit of how many API calls can be made per second, and they usually increase this limit according to the subscription plan you're in. For more context, check out this Shopify API rate limits article. BigCommerce has some info on their FAQs page as well.
2. The size of the file that's being run by Spark Shipping.
Spark Shipping parses the vendor's feed and pushed the updates to your store in parallel. Back to point #1, if the file Spark Shipping is running is large and it's limited by the API rate limit, it is expected for the file to take longer to finish updating compared to other smaller files.
Here are few tips that can help speed up the updates in Spark Shipping:
1. Set the inventory to run on a schedule that best fits your needs, your vendor's updates schedule and and your site's API rate limit. Also, try not to schedule multiple inventory updates to run at the same time.