The purpose of this mixed-methods study was to explore Iranian IELTS candidates' strengths and weaknesses in IELTS Speaking Test in terms of IELTS's four speaking assessment criteria, namely Fluency and Coherence (FlC), Lexical Resource (LR), Grammar Range and Accuracy (GRA), and Pronunciation (Pro). It also aimed to examine the discourse features of the candidates' performances in part 2 of IELTS Speaking across bands 5, 6, and 7. To this end, the oral performances of 59 IELTS candidates from a series of Mock IELTS Tests were collected, re-scored, and subjected to statistical investigation. Additionally, to better understand the performances, we conducted a content analysis on part 2 of the Test. The results of our regression analysis showed that FlC was the greatest predictor of success in IELTS Speaking, followed by GRA and LR, respectively while Pro was found to make the least unique contribution. Furthermore, our content analysis coupled with the application of the Monte Carlo test revealed that as the total scores moved up from 5 to 7, the rate of occurrence of the uncovered faults often declined substantially. It also showed that the association between the frequency of unearthed grammatical complications and the scores 5, 6, and 7 was strong whereas those of the remaining criteria were moderate. Moreover, the most salient obstacle found in the area of FlC, LR, GRA, and Pro, was incorrect connectors and conjunctions, inappropriate word choices, inaccurate simple sentences, and mispronunciations, respectively. The study holds clear implications for IELTS trainers, language teachers, and material developers.