Try asyncRequestProcesssing: true

lib/app.js

@@ -125,6 +125,7 @@ class App {
     this.providerOptions.gasLimit = this.gasLimitString;
     this.providerOptions.allowUnlimitedContractSize = true;
     this.providerOptions.db = memdown();
+    this.providerOptions.asyncRequestProcessing: true;
 
     // Try to launch provider and attach to vm step of
     // either plugin's ganache or a provider passed via options