The documentation suggests `LDAPRetryDelay` is in seconds.

The code appears to treat it as microseconds.

The effect is that Apache sometimes issues rapid bursts of almost simultaneous LDAP search/bind requests.

The bug fix is simply to replace

   apr_sleep(st->retry_delay);

with

   apr_sleep(apr_time_from_sec(st->retry_delay));

(httpd/modules/ldap/util_ldap.c:668).

Here's my research:

Issue:
  LDAP configuration option LDAPRetryDelay...

  The Documentation suggests that the unit is seconds:
    https://httpd.apache.org/docs/2.4/mod/mod_ldap.html#ldapretrydelay

  The code suggests that the unit is microseconds (see code chase, below).

Effect:
  Apache sometimes issues a burst of almost simultaneous LDAP requests.

  (In my organisation, this is "catastrophic" since, if the password is
   incorrect, it appears as N failed login attempts, and the account is
   instantly blocked (after just a single attempt). In practice,
   I've observed N in the region of 5 to 7.)

Configuration option:

  LDAPRetryDelay 5   (for example)

  This sets the retry delay for LDAP connections.

  In the code, this ends up here...

In util_ldap_set_retry_delay (util_ldap.c:2859):

  st->retry_delay = timeout;

  Note... no unit conversion takes place; the code just checks that it's
  a non-negative integer and notes the value for later.

The delay is implemented in httpd/modules/ldap/util_ldap.c:668:

  apr_sleep(st->retry_delay);

  Note... we still appear to have the raw value from the configuration
  file (nominally in seconds).

If you search the code, you will find that apr_sleep is ALMOST ALWAYS
called like this:

  apr_sleep(apr_time_from_sec(XXXX))

  That is, the unit expected is whatever is returned by apr_time_from_sec().

In APR, apr_time_from_sec() is defined like this (apr/include/apr_time.h):

  /** number of microseconds per second */
  #define APR_USEC_PER_SEC APR_TIME_C(1000000)

  .
  .
  .

  /** @return seconds as an apr_time_t */
  #define apr_time_from_sec(sec) ((apr_time_t)(sec) * APR_USEC_PER_SEC)

  So, the result of apr_time_from_sec is in microseconds.

  So the call in util_ldap.c needs to be fixed (as stated at the top).