This here is the shortest time between 2 summons I have found in my log of the Combine (progression) server:
Sebilis:
	Quote:
	
	
		| [Sun Aug 20 22:22:06 2006] froglok ilis knight says 'You will not evade me Pollution!' [Sun Aug 20 22:22:06 2006] Froglok ilis knight staggers.
 [Sun Aug 20 22:22:06 2006] Gaseker staggers.
 [Sun Aug 20 22:22:07 2006] Froglok ilis knight has taken 81 damage from your Bond of Death.
 [Sun Aug 20 22:22:08 2006] Avicenna tells the group, 'i guess the other camp means i can pull the named from here easier'
 [Sun Aug 20 22:22:09 2006] Froglok ilis knight staggers.
 [Sun Aug 20 22:22:09 2006] Gaseker staggers.
 [Sun Aug 20 22:22:11 2006] Avicenna tells the group, 'Incoming Complete Healing to >>> Pollution <<<'
 [Sun Aug 20 22:22:11 2006] Avicenna begins to cast a spell.
 [Sun Aug 20 22:22:12 2006] Keldonis tells the group, 'yeah lets help him get a robe'
 [Sun Aug 20 22:22:13 2006] Froglok ilis knight has taken 84 damage from your Bond of Death.
 [Sun Aug 20 22:22:15 2006] You begin casting Screaming Terror.
 [Sun Aug 20 22:22:16 2006] You tell your party, 'a sebilite golem mezzed'
 [Sun Aug 20 22:22:16 2006] froglok ilis knight says 'You will not evade me Pollution!'
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 The Ruins of Old Paineel:
	Quote:
	
	
		| [Fri Aug 25 01:48:45 2006] Your Screaming Terror spell has worn off of a rock golem. [Fri Aug 25 01:48:45 2006] a rock golem says 'You will not evade me Maliciosa!'
 [Fri Aug 25 01:48:45 2006] You have been summoned!
 [Fri Aug 25 01:48:45 2006] A rock golem hits YOU for 126 points of damage.
 [Fri Aug 25 01:48:45 2006] A rock golem hits YOU for 224 points of damage.
 [Fri Aug 25 01:48:46 2006] A flighty fiend has taken 23 damage from your Splurt.
 [Fri Aug 25 01:48:46 2006] Xonann bashes a rock golem for 10 points of damage.
 [Fri Aug 25 01:48:48 2006] Xonann hits a rock golem for 20 points of damage.
 [Fri Aug 25 01:48:48 2006] A rock golem hits YOU for 135 points of damage.
 [Fri Aug 25 01:48:48 2006] A rock golem hits YOU for 197 points of damage.
 [Fri Aug 25 01:48:49 2006] Rusten tells the group, 'yeah my bad thought iwas casting tap'
 [Fri Aug 25 01:48:49 2006] A flighty fiend bashes YOU for 19 points of damage.
 [Fri Aug 25 01:48:49 2006] Tuazen begins to cast a spell.
 [Fri Aug 25 01:48:50 2006] Xonann hits a rock golem for 36 points of damage.
 [Fri Aug 25 01:48:50 2006] Xonann tells you, 'I am unable to wake a flighty fiend, master.'
 [Fri Aug 25 01:48:50 2006] Taunting attacker, Master.
 [Fri Aug 25 01:48:51 2006] You begin casting Feign Death.
 [Fri Aug 25 01:48:51 2006] A flighty fiend has taken 36 damage from your Splurt.
 [Fri Aug 25 01:48:51 2006] a flighty fiend has been awakened by Maliciosa.
 [Fri Aug 25 01:48:52 2006] A flighty fiend hits YOU for 33 points of damage.
 [Fri Aug 25 01:48:52 2006] You regain your concentration and continue your casting.
 [Fri Aug 25 01:48:53 2006] an elemental wizard begins to cast a spell.
 [Fri Aug 25 01:48:53 2006] Xonann hits a rock golem for 47 points of damage.
 [Fri Aug 25 01:48:53 2006] Xonann hits a rock golem for 47 points of damage.
 [Fri Aug 25 01:48:54 2006] Tuazen hit a flighty fiend for 855 points of non-melee damage.
 [Fri Aug 25 01:48:54 2006] A flighty fiend screams as a magic force rends their flesh.
 [Fri Aug 25 01:48:55 2006] Xonann hits a rock golem for 13 points of damage.
 [Fri Aug 25 01:48:56 2006] a flighty fiend begins to use a rock golem as a living shield!
 [Fri Aug 25 01:48:56 2006] a rock golem says 'You will not evade me Destructioner!'
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 Plane of Hate:
	Quote:
	
	
		| [Fri Sep 01 00:16:04 2006] a loathling lich says 'You will not evade me Velvetmoon!' [Fri Sep 01 00:16:04 2006] a loathling lich begins to cast a spell.
 [Fri Sep 01 00:16:05 2006] A loathling lich's body begins to splurt.
 [Fri Sep 01 00:16:05 2006] A loathling lich looks angry.
 [Fri Sep 01 00:16:06 2006] Taunting attacker, Master.
 [Fri Sep 01 00:16:06 2006] a loathling lich regains concentration and continues casting.
 [Fri Sep 01 00:16:06 2006] You resist the Immobilize spell!
 [Fri Sep 01 00:16:07 2006] A loathling lich has taken 136 damage from your Ignite Blood.
 [Fri Sep 01 00:16:07 2006] A loathling lich has taken 11 damage from your Splurt.
 [Fri Sep 01 00:16:08 2006] a loathling lich begins to cast a spell.
 [Fri Sep 01 00:16:08 2006] Uggme staggers.
 [Fri Sep 01 00:16:08 2006] A loathling lich staggers.
 [Fri Sep 01 00:16:09 2006] a loathling lich begins to cast a spell.
 [Fri Sep 01 00:16:10 2006] Uggme scores a critical hit! (35)
 [Fri Sep 01 00:16:11 2006] Velvetmoon begins to cast a spell.
 [Fri Sep 01 00:16:11 2006] Your feet adhere to the ground.
 [Fri Sep 01 00:16:12 2006] Taunting attacker, Master.
 [Fri Sep 01 00:16:13 2006] Kebarn backstabs a loathling lich for 80 points of damage.
 [Fri Sep 01 00:16:13 2006] A loathling lich has taken 127 damage from your Ignite Blood.
 [Fri Sep 01 00:16:13 2006] A loathling lich has taken 23 damage from your Splurt.
 [Fri Sep 01 00:16:13 2006] a loathling lich begins to cast a spell.
 [Fri Sep 01 00:16:14 2006] Kebarn hits a loathling lich for 22 points of damage.
 [Fri Sep 01 00:16:14 2006] A loathling lich staggers.
 [Fri Sep 01 00:16:14 2006] Kebarn staggers.
 [Fri Sep 01 00:16:14 2006] Kebarn hits a loathling lich for 44 points of damage.
 [Fri Sep 01 00:16:14 2006] Kebarn hits a loathling lich for 17 points of damage.
 [Fri Sep 01 00:16:15 2006] Velvetmoon hit a loathling lich for 571 points of non-melee damage.
 [Fri Sep 01 00:16:15 2006] A loathling lich's mind warps.
 [Fri Sep 01 00:16:15 2006] a loathling lich says 'You will not evade me Velvetmoon!'
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 Of course this doesn't prove the interval can't be shorter, and I could forge it - but I ask everyone to check their log files for any summon interval less than 10 seconds by the same mob (please make sure it isn't 2 mobs of the same name summoning 
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I just analyzed 115 MB of 2005/06 Logfiles from Antonius Bayle and The Combine (Progression):
- filtered lines with "will not evade" into a text file
- opened in OpenOffice Calc as text import, columns separated by space
- created 4 new columns with:
  a) timespan between previous and current summon
  b) timespan between current and next summon
  c) filter column to have value 1 when either timespan is less than 12 seconds
  d) column with the smaller value of a) and b) to hold the smallest timespans between summons
I then looked through all lines that had a value of 1 in column c) and thus an interesting summon time:
The *only* mob I have found pre-Planes of Power with a summon interval of less than 10 seconds was Cazic Thule in the revamped (epics) Plane of Fear: he summons in 6 second intervals.
Am happy to provide logfile extracts with the summon lines - am still looking for older logfiles, but I may have lost them in a HD crash.