The EEOC will be first to tell you that they are overworked and understaffed. At least that's their explanation. I believe that the Agency's poor performance is more a characteristic of the entitlement mentality that most of its employees have, that indolence and delay are endemic to the agency, and that this won't change without a thorough clean-up of the agency's personnel -- from top to bottom.
The worst of the ranks are the employees classified as "Investigators." They are the least trained and the least motivated. Occasionally you will get an investigator who is diligent. Occasionally you will get one who is both diligent and competent. These are the exceptions -- not the rule. EEOC attorneys generally are better performers, as are the EEOC's internal mediators who really work their cases as well as they can.
It takes the EEOC over a year to start most of their investigations. When they get around to investigating a charge, they pursue most of them in a lackadaisical manner and fail to follow up leads. They almost never speak to all the witnesses you name. They rarely uncover witnesses on their own. They ignore relevant evidence. They rush to make no cause findings so they can close out their cases, and make their performance statistics look better. I had one ADA case where one of two co-employers of my client wrote to the other co-employer an email suggesting that they fire the employee under the pretext of canceling the contract between the co-employers because my client suffered from bi-polar disorder "and we can't expect persons with such a condition to be stable employees." The email was actually located in the personnel file that the employer had turned over to the EEOC. The EEOC investigator ignored it -- even though this was direct evidence of the violation of the ADA. The EEOC found there was "no cause" to believe the employer had discriminated unlawfully against my client because of her mental illness.
The delay you are experiencing is not unusual. It is not a sign of anything. Just don't expect much from the EEOC in the end. You can let them complete their investigation, so that the employer's position is "locked-in" before you file a complaint. If you have an aggressive and competent labor and employment attorney who has already nailed down your witnesses with statements showing evidence of the employer's discriminatory treatment you can have him ask for a right to sue letter, and get the EEOC file to see if there is anything valuable in it.