Im wondering how/if we can use pure php coding in elgg? seems like not working....

for example, I need a form...I have to define the form 1st, then use elgg_view_forms...to display the form....instead can I just define my own form? like <form  .......><input ......></input></form>..seems like not working.....and can I also just using pure php to code instead of using elgg's functions/class?

Because I realise that it is slow loading the elgg_view_forms and other stuff is slow too...