The Asterisk base code cannot handle any calls. It needs to be configured, typically with a dialplan. FreePBX provides such a dialplan, but it is a very long and complicated one, so it would be possible to handle a higher call attempt rate if you provided your own simplified dialplan. That’s not normally an issue, but I do remember one case where someone using Asterisk on its own was generating so many calls that the machine ran out of power when just one extra dialplan line was added to a very short one.
I think the main performance impacts of FreePBX are in terms of call attempt rates, because of the size and complexity of its diaplans, and in media throughput, particularly because it tends to monitor media for DTMF, so media nearly always has to flow through it. However, hand constructed dialplans could achieve the same complexity, or involve slow running operations, and hand designed configurations may also not allow effective use of direct media.