SQL - Slow String Comparison

Question

I'm having a problem that I really don't understand. I have a SQL query that takes about 3 s to compute and returns a table with 2500 lines. If I try to narrow the results with any of the following simple conditions

column_something IS NOT NULL
column_something = 'string_a'
column_something BETWEEN 'string_a' AND 'string_c'

the query becomes much slower (more than 10 min, sometimes) to return 100 lines, or so. Does anyone have an explanation for this?

In my company, we use a software where every action is performed through a GUI and the information is stored on an Oracle DB. We’re only able to access de DB to retrieve information using SQL SELECTs.
The query is the following:

1   SELECT              
2       mpno,           
3       mp_rev,         
4       taskcardno,         
5       task_rev,           
6       docno,          
7       doc_type,           
8       doc_rev,            
9       ac_registr,         
10      pre_post            
11  FROM                
12      (SELECT         
13          DOC.mpno,       
14          DOC.mp_rev,     
15          DOC.taskcardno,     
16          DOC. task_rev,      
17          DOC.docno,      
18          DOC.doc_type,       
19          DOC.doc_rev,        
20          TC.ac_registr,      
21          MAX(CASE        
22              WHEN (SELECT P.amount_int FROM parameters P WHERE P.parameter = '1428') =  0 THEN   
23                  CASE
24                      WHEN TC.event_status IN ('O', 'A', 'B', 'J') THEN 'PRE'
25                      WHEN TC.event_status IN ('F', 'T', 'V', 'C') THEN  'POST'
26                  END
27              ELSE    
28                  CASE
29                      WHEN TC.event_status IN ('O', 'A', 'B', 'J', 'N', 'E', 'I', 'Y', 'R') THEN 'PRE'
30                      WHEN TC.event_status IN ('F', 'T', 'V', 'C') THEN  'POST'
31                  END
32          END) pre_post       
33      FROM            
34          (SELECT     
35              DH.docno,   
36              DH.doc_type,    
37              DH.revision doc_rev,    
38              DH.docno_i, 
39              TC.mpno,    
40              TC.mp_rev,  
41              TC.taskcardno,  
42              TC.taskcard_verno_i,    
43              TC.task_rev 
44          FROM        
45              doc_header DH   
46              JOIN    
47              (SELECT DISTINCT    
48                  DH.docno,
49                  DH.doc_type,
50                  MMP.mpno,
51                  MMR.revision_no mp_rev,
52                  MT.taskcardno,
53                  MTV.taskcard_verno_i,
54                  MTV.revision_no task_rev
55              FROM    
56                  msc_maintenance_program MMP
57                  JOIN msc_taskcard MT ON MMP.mpno_i = MT.mpno_i 
58                  JOIN msc_taskcard_version MTV ON MT.taskcardno_i = MTV.taskcardno_i 
59                  JOIN msc_mprev_taskcards MMT ON MTV.taskcard_verno_i = MMT.taskcard_verno_i 
60                  JOIN msc_mp_revision MMR ON MMT.revisionno_i = MMR.revisionno_i 
61                  JOIN msc_item MI ON MTV.taskcard_verno_i = MI.taskcard_verno_i
62                  JOIN msc_taskcard_version MTV ON MT.taskcardno_i = MTV.taskcardno_i 
63                  JOIN msc_item MI ON MTV.taskcard_verno_i = MI.taskcard_verno_i
64                  JOIN event_effectivity_link EEL ON MI.itemno_i = EEL.event_key AND EEL.event_type = 'TI'
65                  JOIN db_link DB ON EEL.effectivity_linkno_i = DB.source_pk AND DB.source_type = 'EFL' AND DB.destination_type = 'DO'
66                  JOIN doc_header DH ON DB.destination_key = DH.docno_i AND DH.release_state <> 'I'
67              ) TC ON DH.docno = TC.docno AND DH.doc_type = TC.doc_type   
68          WHERE       
69              DH.release_state <>'I'  
70          ) DOC       
71          LEFT JOIN       
72          (SELECT     
73              MTV.taskcard_verno_i,   
74              DH.docno_i, 
75              WH.ac_registr,  
76              WEL.event_status    
77          FROM        
78              msc_maintenance_program MMP 
79              JOIN msc_taskcard MT ON MMP.mpno_i = MT.mpno_i  
80              JOIN msc_taskcard_version MTV ON MT.taskcardno_i = MTV.taskcardno_i     
81              JOIN msc_mprev_taskcards MMT ON MTV.taskcard_verno_i = MMT.taskcard_verno_i     
82              JOIN msc_mp_revision MMR ON MMT.revisionno_i = MMR.revisionno_i     
83              JOIN msc_item MI ON MTV.taskcard_verno_i = MI.taskcard_verno_i  
84              JOIN event_effectivity_link EEL1 ON MI.itemno_i = EEL1.event_key AND EEL1.event_type = 'TI' 
85              JOIN applicability A ON EEL1.effectivityno_i = A.effectivityno_i AND A.ref_type = 'AC_INT'  
86              JOIN aircraft AC ON A.ref_key = AC.aircraftno_i     
87              JOIN db_link DB ON EEL1.effectivity_linkno_i = DB.source_pk AND DB.source_type = 'EFL' AND DB.destination_type = 'DO'   
88              JOIN doc_header DH ON DB.destination_key = DH.docno_i AND DH.release_state <> 'I'   
89              LEFT JOIN doc_replacement DR ON DH.docno_i = DR.docno_i AND DR.replacement_code IN ('SB', 'CB') 
90              LEFT JOIN doc_signoff_tree DST ON DH.docno_i = DST.ref_docno_i OR DR.ref_docno_i = DST.ref_docno_i OR DH.docno_i = DST.docno_i OR DR.ref_docno_i = DST.docno_i  
91              LEFT JOIN event_effectivity_link EEL2 ON (DST.docno_i = EEL2.event_key OR DST.ref_docno_i = EEL2.event_key) AND EEL2.event_type = 'DO'  
92              LEFT JOIN wo_event_link WEL ON EEL2.event_key = WEL.event_key_parent AND WEL.event_type = 'DOC_EFF' AND WEL.pending_status = 0  
93              LEFT JOIN wo_header WH ON WEL.event_perfno_i = WH.event_perfno_i AND WH.ac_registr = AC.ac_registr  
94          WHERE       
95              WEL.event_status IS NOT NULL    
96          ) TC ON (DOC.docno_i = TC.docno_i AND DOC.taskcard_verno_i = TC.taskcard_verno_i)       
97      GROUP BY            
98          DOC.mpno,       
99          DOC.mp_rev,     
100         DOC.taskcardno,     
101         DOC. task_rev,      
102         DOC.docno,      
103         DOC.doc_type,       
104         DOC.doc_rev,        
105         TC.ac_registr       
106     )           
107 WHERE               
108     ac_registr = 'TPO'

If I run lines 1 to 106, it returns the 2500 lines after 3s as I mentioned.
If I add lines 107 and 108, it takes around 10 min to return 100 lines.
ac_registr is indexed as well as every column used on JOINs.
I know this is quite specific and finding help would be a long shot, but being a self-taught noob I was looking for a general explanation of the rationale behind this, because I just don’t get it.
Thank you.