diff --git a/scripts/base/init-bare.zeek b/scripts/base/init-bare.zeek
index af5e9b08fb..b69880db55 100644
--- a/scripts/base/init-bare.zeek
+++ b/scripts/base/init-bare.zeek
@@ -5581,6 +5581,35 @@ const digest_salt = "Please change this value." &redef;
 ## :zeek:see:`find_all_ordered` BIFs.
 const max_find_all_string_length: int = 10000 &redef;
 
+## How many rounds to go without checking IO sources with file descriptors
+## for readiness by default. This is used when reading from traces.
+##
+## Very roughly, when reading from a pcap, setting this to 100 results in
+## 100 packets being processed without checking FD based IO sources.
+##
+## .. note:: This should not be changed outside of development or when
+##    debugging problems with the main-loop, or developing features with
+##    tight main-loop interaction.
+##
+## .. zeek:see:: io_poll_interval_live
+const io_poll_interval_default = 100 &redef;
+
+## How often to check IO sources with file descriptors for readiness when
+## monitoring with a live packet source.
+##
+## The poll interval gets defaulted to 100 which is good for cases like reading
+## from pcap files and when there isn't a packet source, but is a little too
+## infrequent for live sources (especially fast live sources). Set it down a
+## little bit for those sources.
+##
+## .. note:: This should not be changed outside of development or when
+##    debugging problems with the main-loop, or developing features with
+##    tight main-loop interaction.
+##
+## .. zeek:see:: io_poll_interval_default
+const io_poll_interval_live = 10 &redef;
+
+
 global done_with_network = F;
 event net_done(t: time)
 	{ done_with_network = T; }
diff --git a/src/const.bif b/src/const.bif
index f28429b33c..4d793d82c1 100644
--- a/src/const.bif
+++ b/src/const.bif
@@ -11,6 +11,9 @@ const exit_only_after_terminate: bool;
 const digest_salt: string;
 const max_analyzer_violations: count;
 
+const io_poll_interval_default: count;
+const io_poll_interval_live: count;
+
 const FTP::max_command_length: count;
 
 const NFS3::return_data: bool;
diff --git a/src/iosource/Manager.cc b/src/iosource/Manager.cc
index 537965674a..1cf2d9e599 100644
--- a/src/iosource/Manager.cc
+++ b/src/iosource/Manager.cc
@@ -105,6 +105,7 @@ Manager::~Manager()
 void Manager::InitPostScript()
 	{
 	wakeup = new WakeupHandler();
+	poll_interval = BifConst::io_poll_interval_default;
 	}
 
 void Manager::RemoveAll()
@@ -401,12 +402,8 @@ void Manager::Register(PktSrc* src)
 	{
 	pkt_src = src;
 
-	// The poll interval gets defaulted to 100 which is good for cases like reading
-	// from pcap files and when there isn't a packet source, but is a little too
-	// infrequent for live sources (especially fast live sources). Set it down a
-	// little bit for those sources.
 	if ( src->IsLive() )
-		poll_interval = 10;
+		poll_interval = BifConst::io_poll_interval_live;
 	else if ( run_state::pseudo_realtime )
 		poll_interval = 1;
 
diff --git a/src/iosource/Manager.h b/src/iosource/Manager.h
index 6fad1adcc5..dbfba845e7 100644
--- a/src/iosource/Manager.h
+++ b/src/iosource/Manager.h
@@ -216,7 +216,7 @@ private:
 	int zero_timeout_count = 0;
 	WakeupHandler* wakeup = nullptr;
 	int poll_counter = 0;
-	int poll_interval = 100;
+	int poll_interval = 0; // Set in InitPostScript() based on const value.
 
 	int event_queue = -1;
 	std::map<int, IOSource*> fd_map;