Parallelize URL indicator checks

docs/iocs.md

@@ -37,8 +37,10 @@ export MVT_STIX2="/home/user/IOC1.stix2:/home/user/IOC2.stix2"
 ## Network Access
 
 When checking URL indicators, MVT follows recognized shortened URLs with an
-HTTP `HEAD` request. The following environment variables control these
-requests:
+HTTP `HEAD` request. URL checks are deduplicated and run concurrently, with at
+most 20 requests in progress at a time. Redirects within an individual URL
+chain are still followed sequentially. The following environment variables
+control these requests:
 
 - `MVT_NETWORK_ACCESS_ALLOWED` enables or disables network requests. It defaults
   to `true`. Set it to `false` to prevent MVT from attempting to resolve
@@ -73,4 +75,3 @@ You can automaticallly download the latest public indicator files with the comma
 
 Please [open an issue](https://github.com/mvt-project/mvt/issues/) to suggest new sources of STIX-formatted IOCs.
 
-