If the main purpose of using voice typing is to improvement in the speed of posting, then I am afraid it is not the right method. It may even take more time to first voice type a post and then do all the spell checks and rectify grammatical mistakes.
I agree that with voice typing the speed of posting will further slow down. The voice tying may help those members who are lazy enough to write the posts themselves using their fingers, but I don't think that someone can express his/her ideas very well with the help of voice typing. You'll still be reading the forum posts as a text based posts and with the voice typing you'll be replying to those posts with your voice which will limit your imagination that you can have with traditional way of typing.
Voice typing can be useful for those people who have severe muscle and joint problems but the ones who are healthy will become more lazy because of voice typing. I also agree with you that the ones who will use voice typing will have more spelling and grammatical mistakes, and then they'll have to fix those mistakes one by one, and which will be very time consuming task for them.
We never know that many posters may already be using this voice typing feature present inside the browser as we see a lot of posts with grammatical mistakes and lot of other posts with spelling mistakes.
Also if you are using a mobile to make a post, voice typing becomes even more easier, but again the drawbacks of voice typing do exist.
Yes, there could be some posters who might be using voice typing if they're basically smartphone users but the posts with grammatical mistakes and other spelling mistakes can be because of other reasons as well. Some users who aren't good in English may have grammatical mistakes and spelling mistakes in their posts, and there could also be those users who want to write their posts as soon as possible, and post it without giving it a reading. I personally don't post using my smartphone because I don't like that way of posting as of now, but still sometimes I make grammatical mistakes. I believe those mistakes take place only when we don't read our written posts. The worst thing is that the ones who will make posts using voice typing will make far more grammatical and spelling mistakes then the ones who aren't good in English.
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The voice typing feature is already available in your browser and you can easily use it. It has been explained in the above post too.
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No it is not. You assume that we all use mobile iOS/Android platforms or have speech recognition capabilities in our operating systems, which is not true.
I agree with you that we all don't use iOS and Android or even smartphones that have those capabilities. If I'm not wrong then most members of the forum still type their posts using their own personal computer or laptops and those systems don't really have such capabilities built it them. The pc users will have to use external microphones in order to utilize voice typing extensions or applications, while the laptop users may get a little bit of ease because the laptops have built in microphones. But, still they will have to rely on third party software, extensions, and websites in order to utilize voice typing.