It is not clear that "solid prosecutions" are being dismissed. Any sensible prosecutor, faced with a delay problem, will go through his files and drop the weakest cases. If that's not enough, (s)he will then drop the cases where the amount of time the case is expected to consume is disproportionate to the gravity of the offence (the length of trials is related to the complexity of the evidence more than to the seriousness of the crime, except that there seems to be a rule throughout the legal community that trials for very serious crimes must take at least a few days of court time). 

The last cases to be dropped in a delay crunch will be serious crimes trials where the Crown has a high likelihood of conviction.