Oh I got that wrong didn't I? Oops.
2500 RPM is actually 2500 / 60 Rev per second, that is 41.6 Hz.
Well in that case measuring the period is quite achievable and to a high resolution. At low speeds measuring the period is more accurate because obviously at 41 Hz you potentially have a 1/41 error if the count is out by one (2.4%).
If you measure the period you get an instantaneous figure (well, after 1/41 of a second elapses) rather than having to wait a whole second. You could average a couple of them to smooth out variations.