"S.F.f 13" ... Are you referring to SF Pro or SF Audio Studio?
What are are the MP3 encode settings?
Plug-ins can slow things down a lot. I normally render a PCM file, then encode MP3s from that.
I recall SF Pro 12 slow rendering MP3 . So I checked a one minute stereo PCM file: Pro 13 (build 100): 5.2 sec.s, Pro 12 (build 170) was a half-second slower; VP-16 was the same as SF Pro 13; SF Pro 10 was one second faster than SFP 13.
My MP3 encode parameters were all set to 256kbps, 44.1KHz, CBR, Stereo, Max quality. There are no plug-ins are other processors enabled on the apps. PC: Win 10 w/ 32 MB RAM.
Another option is a third-party MP3 encoder. I recently stated in another thread about the WinLAME RC3. That rendered the test file almost twice as fast as SF Pro 10. RC3 also has a HPF parameter in the UI and some other useful parameters can be customized in the presets file/script.
IMHO though, the Fraunhofer encoder sounds slightly better at low bitrates. I don't know if SF Audio Studio has the Fraunhofer encoder, as does SF Pro.
The LAME encoder is not available in SF. It can be added, but I recall it was difficult and required registry tweak and such, so I have never tred it. I usually render a PCM master beforehand anyway, so using a third-party UI is no big deal.