I understand that Maemish but keep in mind that Leste is not Fremantle and in 2025, browsing heavy websites is not possible anymore and we can't compare with how it was going on with Fremantle 10 years ago.
N900 is still a fantastic device, it is a miracle almost everything still works with very recent stuff. But you can't expect good web performances on the same device with websites X100 heavier in 2025.

Apart web browsing, you can expect no slowdown with Leste, really. (Of course if you use proper apps optimized and built for N900).

As well, keep in mind that most (if not all) slowdowns users have seen with Leste during the last couple of years were due to slow SDcards...

And about what you said at 5, yeah totally agree