In the end, however, he is right. Subjective descriptions of a device becoming "slow" do not help diagnose any possible issues. In any case:

Which memory? Remember, the N900 has multiple memory types:
- RAM
- Rootfs (256MB where the OS is stored)
- eMMC which stores
> /home - where all of your user settings are stored
> /opt - where all installed applications go
> swap - the 768MB Nokia adds to the ram to say 1GB
- SD Card (which may or may not be there.)

Filling the rootfs to 95-100 is known to cause problems, including being unable to do pretty much anything. The only issue I could see is if you have an application open so long that it begins to use all the ram and force things out to swap.