Sony Ericsson :: F305 / Responding Really Slow

Just got a debranded f305 and well it came with a 512mb card.I put my own 1gb card full with about 250 m4a songs now the phone is VERY SLOW when i enter music in file manager and slow in the media player takes like 3-4 seconds for it to start playing a selected song from list and like 5 seconds when i click the next in the player.now i understand this phone supports up to 4GB so if its so slow with 1 GB then how can 4gb even be supported.my own w910i runs smooth with much more music in file manager and is always instant in selecting and playing music in the player.... the f305 is a low-mid range phone but even the k610 has a fast file manager and music player so what gives?any ideas how to reduce the lag? or why is it happening?