Performance issue with Manticore Search

Hello, the response time of my index on Manticore Search has increased a lot recently (some requests take several seconds). There are about 40 million documents currently indexed. How can I increase the performance of Manticore Search? Is there any configuration I can do that would improve it? I have put below my current configuration.

index my_index
{
    type                = rt
    path                = /mnt/.../manticore/data/my_index

    rt_field            = field1
    rt_field            = field2
    stored_fields       =

    rt_attr_timestamp   = attr1
    rt_attr_string      = attr2
    rt_attr_string      = attr3
    rt_attr_string      = attr4

    charset_table       = non_cjk
    ngram_len           = 1
    ngram_chars         = cjk

    rt_mem_limit        = 2048M
}

searchd
{
    listen              = 127.0.0.1:9312
    listen              = 127.0.0.1:9306:mysql41
    log                 = /var/log/manticore/searchd.log
    pid_file            = /var/run/manticore/searchd.pid
    seamless_rotate     = 1
    preopen_tables      = 1
    unlink_old          = 1
    net_workers         = 2
    binlog_path         = /mnt/.../manticore/data
    rt_flush_period     = 3600
}