Thinking Sphinx performance problem

Hi all, for first sorry for my english.

I've got a problem with RoR and Thinking Sphinx gem.

I mean: everything works fine but when I try to get a search result passing a lot of parameter values through Active Record and TS API my application slow down and become unusable.

I need to send queries on a MyISAM table with over 600.000 records

There's a query field that I can match with 0 or N values If I set 0 or few values the query is fast. If I set 1000-2000 or more values for this field the query occurs over 10 minutes.

I'm not sure about the query slowness because the problem occurs only when I try to get (for example) the total_count value.

The query is something like: Model.search match_mode: :extended, page: page, per_page: per_page, :order => :updated_at, :sort_mode => :desc

If I look in the searchd.query.log file I can find this informations

[Tue Jan 14 09:42:08.394 2014] 774.749 sec [ext/2/attr- 64637 (0,20)] [digitalasset_core,digitalasset_delta] @has_countries ,98, | ,88, | ,ALL, | , , @has_12ncs ,851322311100, | ,851304211100, | ,851340001000,

,851343701000, | ,851343801000, | ,851343901000, | ,851344001000, |

,858758938790, | ,858759001790, | ,858759038790, | ,858759701790, | ,851343801020, | ,851343701020, | ,851344001020, | ,851343901020, | ,858759901740, | ,858759701740, | ,857542038000, | ,857594938000, | ,857542438000, | ,850120596000, | ,850120696000, | ,850120796000, | ,850122601020, | ,850122701000, | ,850122701010, | ,850122801000, | ,850122801020, | ,851315001000, | ,851321801000, | ,851535101040, | ,858600015000, | ,858600096000, | ,858600096010, | ,850122901010, | ,850123001000, | ,850123001020, | ,851315001010, | ,850123001010, | ,851328301000, | ,851328301010, | ,851328401010, | ,851385101080, | ,854001638000, | ,854071038080, | ,854071038090, | ,854074638070, | ,856070001080, | ,856079838070, | ,857532008400, | ,857532108400, | ,857565108200, | ,857565738010, | ,857575738010, | ,857576838010, | ,857582338080, | ,857584738000, | ,857586738000, | ,857586838080, | ,857588838080, | ,857597838010, | ,857599938030, | ,854060038010, | ,854070038010, | ,854080038010, | ,857580000110, | ,856087638010, | ,857597838020, | ,857586838020, | ,857576838020, | ,854076738020, | ,857597838030, | ,857587838010, | ,856010038020, | ,856087838020, | ,857500038020, | ,857500138020, | ,857500238020, | ,857500338020, | ,854020038020, | ,856010048020, | ,856010058020, | ,857500348020, | ,857500358020, | ,857500368020, | ,857580001100, | ,851330038000, | ,851330138000, | ,851330238000, | ,851330338000, | ,851330438000, | ,851330538000, | ,851330601000, | ,851330701000, | ,851330801000, | ,851330901000, | ,851331101000, | ,851331201000, | ,851331301000, | ,851331401000, | ,851331501000, | ,851331838000, | ,851331938000, | ,851338038000, | ,851338138000, | ,851338201000, | ,851338301000, | ,851338401000, | ,851338501000, | ,851345638000, | ,851345738000ΓΏ

the 12NCS are the values that I want to match in order to limit the resultset.

Can anybody help me? Thanks in advance.