REP-4388: Add notes on performance problems. (#18)

* add performance note

* remove bullet

Author: FGasper

README.md

@@ -229,6 +229,12 @@ Any collection metadata mismatches will occur in a task with the type '`verifyCo
 
 In this case, '`failed_docs`' contains all the meta data mismatches, in this case an index named '`x_1`'.
 
+# Performance
+
+The migration-verifier optimizes for the case where a migration’s initial sync is completed **and** change events are relatively infrequent. If you start verification before initial sync finishes, or if the source cluster is too busy, the verification may freeze.
+
+The migration-verifier is also rather resource-hungry. To mitigate this, try limiting its number of workers (i.e., `--numWorkers`), its partition size (`--partitionSizeMB`), and/or its process group’s resource limits (see the `ulimit` command in POSIX OSes).
+
 # Known Issues
 
 - The verifier may report missing documents on the destination that don’t actually appear to be missing (i.e., a nonexistent problem). This has been hard to reproduce. If missing documents are reported, it is good practice to check for false positives.