Is it just a faster way of encoding vbr files or is there any other differences than not using fast mode?

I guess you are talking about LAME encoder, than take a look here:

http://lame.cvs.sourceforge.net/*checkout*/lame/lame/USAGE

From the WIKI



You really asked the question with far too little detail... Nobody will be exactly sure, what exactly you are asking...

Somebody quoted an old post that said the --vbr-new method is not recommended but it has been recommended since the last release of LAME (3.97). Its the default method when using the -V levels on 3.98. Apparently its doing something right if it receives the praise of developers and hydrogenaudio members. Fast mode is simply a quicker way of encoding files with the LAME encoder.

No they didn't. The first reply was referring to an old "-f" option which was for fast but low quality CBR encoding.

I was supposed to be asking if the speed of the preset had any negative effect on the quality. I also should have mentioned my question stemmed from foobar2000's option for the tickbox with "--vbr-new". If I'm using the 3.98.2 encoder and I leave that unticked, it does it anyways because it's the default?

Yes I think it will use --vbr-new anyway as it's the default. If you want to force it to use the older --vbr-old I think you'd have to set a custom converter profile for it (click "add new" and then select "custom" for the encoder in the foobar converter options section.)

As for the "fast" reference I think that this came about because it was faster than the old vbr algorithm so in the old "--preset" method of vbr quality selection they used the modifier "fast" to specify it's use. For example "--preset standard" used the old vbr algorithm whereas "--preset fast standard" used the new vbr algorithm though each still targeted the same "standard" vbr quality. Lately the "--preset" method of specifying vbr target quality has been replaced with the simpler "-Vx" method, so for example the old "--preset standard" would be equivalent to "-V2 --vbr-old" and "--preset fast standard" would be equivalent to "-V2 --vbr-new" or more simply just "-V2" as the --vbr-new is now the default.

As for the original question as to what "fast" mode (as per the meaning --vbr-new) does. It controls the part of the encoding algorithm that selects what bit rate is required to encode a given frame to achieve the targeted quality level. You should not think of "--vbr-new" as faster but lower quality version of "--vbr-old" but rather as a completely new algorithm which just happens to run a little faster than the older algorithm.