Well Caballero, good for you, but it misses even more security, reliability, and performance patches than current MicroB version used on Nokia N900. If all you care about is speed, go snort that. Minefield is dead though.

Usability wise it'd suck arse on N900 because Maemo 5 isn't using the stylus by default whereas your OS -Maemo 4.x- is.

For N900 there are other options to increase (browser) performance, and tons of threads about it. Personally, I opt for Opera Mobile for performance.

Fennec has nothing to do with all of this. It has a different UI, intended for different usage of web browsing. Fennec is still very much in development (just like Mozilla Firefox 1.x was, or the Mozilla Milestones were), and Fennec was never intended to perform great on your hardware hence it is of no surprise it doesn't perform well on your hardware.

If Minefield makes you float your boat, congratulations. However, it is a security hazard to use such out of date software with known security vulnerabilities.