A developer at Symbian Forums explains why the newer Nokia S60 and Sony Ericsson devices based on Symbian OS 9.x take a longer time to load and consume a lot of RAM.

Symbian 9.x phones use NAND flash chips as their “ROM” (where the OS and built-in apps are stored). NAND memory does not support Execute-In-Place (XIP), unlike the previously used NOR chips. This means the OS and the built-in applications are loaded to RAM for execution. The result is slower startup and large RAM consumption, because a lot of ROM-based programs run all the time (not only those shown in task managers).

The advantage: NAND is cheaper. All Symbian 9.x (UIQ3 and S60/3) devices use NAND chips. The current phones have a minimal amount of RAM to just satisfy this feature.