TTFB is totally dependent on server load and available bandwidth.
If your server is heavily loaded (lots of requests per second) then your TTFB will rise exponentially. This effect is multiplied by lower server spec and available bandwidth.
Spreading the load over multiple sub domains may or may not have any effect, this depends on how you actually technically do it:
IF you use multiple sub domains linked to the same server, then expect little or no difference.
However if you spread this load over multiple sub-domains pointing do different servers then i could see potential for a lowering of your TTFB - this is how CDN's work.
HOWEVER it is likely that this high TTFB is caused by another factor such as scripts running on every page load - i would investigate the cause before trying to solve it in this way.