Overclocking is a must so it should be made as easy as possible.

Totally agree but that's not so easy with a mainline up to date kernel...it means providing for each kernel update, 2 different versions of it i think.
My overclock method can't survive between 2 kernel updates most of the time.
We must ask to the dev team. But definitely overclock is essential from my point of view too, especially for gaming, streaming and multitasking. (like Fremantle in fact)

Sadly even with overclock, 2 GB of swap, expanded SD etc. - still Leste is slow like impossible on my N900. Just not usable. Interface is very very laggy, everything (even UI navigation) is so slow and takes lot of time.

I tried three different SD cards and three different N900s - the same

I don't know what's going wrong but even with no overclock the UI is not supposed to be laggy.
Did you try different SDcards just in case ?
Are you up to date with chimaera-devel ?

Note: remember that from the last image swap is activated automatically and use emmc 768MB swap partition. You don't need to create a swap file anymore.

Hah, I wanted to write "I tried three different SD cards" and wrote "SIM cards" instead

I'll try latest image then, but I always used 2 GB swap and UI was laggy anyway On three N900s

I haven't found the UI that laggy and tweaking transitions made a big difference too. It would be good to have transition tweaks to be included the image. Haven't yet tried the latest image.

I wonder if there is something using the ram and cpu on the back, dredlock. Like with fremantle the Opera browser feom devel repo left browserd to work on the back so that it made the device unusable unless rebooted. Do you have conky? With that it is easy to see what stays on and is making device to be laggy.

Not sure if there is already on the latest image the loading bar to show when an app is in use. That is why I have hoped to have Loadapplet on Leste cause it showed on the top bar that there is ram and cpu in use and should just wait when it dropps before trying to do anything else. That helped with Opera browser on fremantle when opening pages you knew to wait before trying to do something else.

Ok, if you're sure you have followed the right process maybe something is wrong with last image(s).
The 20230618 image works well (i even tried it on a very slow SDcard with no troubles). Maybe you could try this one to see if things work better ?

How about USB, does host mode works like it worked with Maemo 5 with patches:
https://talk.maemo.org/showthread.php?t=65232
example can I connect memory-stick or usb-mouse to N900?

Good question, might be possible in theory and really cool.
But that's a kind of tricky hack and not sure it is a priority for devs.
Maybe someone could help for this ?