LOAD DATA INFILE blocks on concurrent use against MyISAM tables
Concurrent LOAD DATA INFILE on MyISAM tables are limited by mutex congestion due a global Key Cache. To alleviate lock congestion, MyISAM tables can be set to have their own Key Cache.

SET GLOBAL isam_cache.key_buffer_size=8*1024;
SET GLOBAL isam_cache_two.key_buffer_size=8*1024;
CACHE INDEX log_isam IN isam_cache;
CACHE INDEX log_isam2 IN isam_cache_two;