Are your tag pages including the excerpt or the content of the post?
For showing the_content you could do something like the following in your functions.php
function reading_time_remove_tag_archive() {
if ( is_tag() ) {
global $reading_time_wp;
remove_filter('the_content', array($reading_time_wp, 'rt_add_reading_time_before_content'));
}
}
add_action( 'wp', 'reading_time_remove_tag_archive' );
Or if using the_excerpt you could do:
function reading_time_remove_tag_archive() {
if ( is_tag() ) {
global $reading_time_wp;
remove_filter('get_the_excerpt', array($reading_time_wp, 'rt_add_reading_time_before_excerpt'), 1000);
}
}
add_action( 'wp', 'reading_time_remove_tag_archive' );