From version SVN:998 the importer ( feedcron.php ) contains a high load protection.

Required: linux OS, php > 5.1.3

Running the importer  on a system with DF_LOAD_CHECK set to true with a load above DF_LOAD_MAX wil abort using the web interface or wait DF_LOAD_WAIT running from the command line

default values:

define ('DF_LOAD_CHECK',true);
define ('DF_LOAD_WAIT',60);
define ('DF_LOAD_MAX',10);

The load is not checked during the feed configuration.

The load is no checked after the import of a feed started. Importing a large feed might raise the load far above the max.

Modify/Add these defines in your administrator/components/com_datafeeds/cron/feeds.php

How high is too high?

really depends on your system, on a single CPU server a load of 10 is high, on a server with four quad core cpu's it might be now problem.

The tool shows the system load on the bottom of the admin panel (svn:1009). Suggestion DF_LOAD_MAX is your average load plus two. (no guarantees, ask your webhosting company before you get yourself in trouble.)