When blocking initialization is specified, use a minimum of 16 connection creation threads.

src/main/java/com/zaxxer/hikari/pool/HikariPool.java

@@ -137,8 +137,8 @@ public final class HikariPool extends PoolBase implements HikariPoolMXBean, IBag
       this.houseKeeperTask = houseKeepingExecutorService.scheduleWithFixedDelay(new HouseKeeper(), 100L, housekeepingPeriodMs, MILLISECONDS);
 
       if (Boolean.getBoolean("com.zaxxer.hikari.blockUntilFilled") && config.getInitializationFailTimeout() > 1) {
-         addConnectionExecutor.setCorePoolSize(Runtime.getRuntime().availableProcessors());
-         addConnectionExecutor.setMaximumPoolSize(Runtime.getRuntime().availableProcessors());
+         addConnectionExecutor.setCorePoolSize(Math.min(16, Runtime.getRuntime().availableProcessors()));
+         addConnectionExecutor.setMaximumPoolSize(Math.min(16, Runtime.getRuntime().availableProcessors()));
 
          final long startTime = currentTime();
          while (elapsedMillis(startTime) < config.getInitializationFailTimeout() && getTotalConnections() < config.getMinimumIdle()) {