If the overall plot/subplots didn't drag as much, most readers' responses would be much less negative, that's for sure—one can argue Gosho's just rehashing stuff, but there's just rehashing stuff, and then there's rehashing stuff and continually doing so without resolution.
From File 768 (her intro) she seemed to know Ran, and Conan, suspecting he was the shrunken Shinichi. It especially began to demand an answer after Mystery Train. The Shinichi/Ran/Masumi flashback is the key to pretty much all the questions about Masumi. Will it live up to the hype? It's really tough to do so—look at Following Holmes/Holmes' Revelation (File 743–File 752/Episode 616–Episode 621). Even in DC, as long as it is, nearly five years of waiting (you can say nearly three, as well) can lead to disappointment. The question of the Bourbon arc that took its entirety to answer was the "Who is Subaru Okiya?" question. We had to wait from September 10th, 2007 (Release of File 622) until June 2nd, 2014 (Release of File 898) for the answer. Some probably figured it out upon his introduction and never wavered from that position. When we got Scarlet Showdown... well... yeah (I personally liked it, for what it is, but, man, do I get why a lot of people don't). Is it assured of disappointment? No. However, it is probable. Still, when it happens, the plot can get going—there are important questions about Masumi that have remained unanswered since March 7th, 2011 (Release of File 768), and they've held the plot up, especially after Mystery Train.