Okay, I tried it again. It was much slower than Mobipocket Reader at pagination, and turned pages slower as well.
My problem with adding uBook is that there are vast numbers of 7, 8, 9 year old devices out there that are still perfectly functional. uBook won't run well on them becuase of the older, weaker CPUs.
I used µBook reader on a variety of devices, encompassing various version of WM and WinCe, and it always worked well.
The only case where page turns were not instantaneous was with my Toshiba E800 with the processor on minimum speed (100Mhz instead of 400, with the processor having to manage a VGA screen).
It was very fast even with HP 6315 or 1910, which were very slow device.
Perhaps the problem only lies with Mips or SH3/SH4 processor. But these devices are very rare today, arm dominates the market since ages.
The great advantage of µBook reside in its customizability, allowing it to be adapted to every device and display size -and every taste-. I used it successfully on some weird CE devices like the Samsung Nexio tablet for example.