When do the previous owner need to handover the house keys to the new owner ? Surely not after signing SPA, right, since he has only received 10% ?

usually. when balance 90% received & all bills have been clear.
u pass the keys, house in 'as is where us conditon' as agreed... the lawyer pass u 90% cheque.

again. read what has been agreed in SPA....

generally is either

1. upon payment of balance purchase price + late payment interest (if any) + apportion the outgoings/bills

or

2. both parties agree for an early handover (whether subject to rental or not is entirely depend on both parties)

i ady kena progressive interest for the first payment bank made to previous owner bank.
now wait for no.1 you said above.
do i need to push my lawyer to hasten the process so that i dont have to keep paying progressive interest?

interests like these are not called "progressive interest", they are just called "late payment interest" but according to you, if you are paying late payment interest for the redemption sum, that means you are already in your Extended Completion Date. You will continue paying interest until the settlement of the Balance Purchase Price which should be maybe close to 2 months from now.

i think you better ask your solicitor to explain to you why are you paying late payment interest and whose fault is it that the disbursement of the loan is delayed?

owh i see..
last time they claim bcoz previous owner using gov loan,
need at least 4 month to complete disbursement process.
now almost 4 month ady.

I think what you mean is

1. bank already release redemption sum to settle owner's bank loan.

2. now lawyer is in the midst of doing the reassignment/discharge

3. now you have to pay interest to your bank based on the redemption sum.

usually it takes about 1month or so from date of redemption release to the date of full release of balance loan sum.

Any longer than 5 weeks then it's time to ask ur lawyer whats the delay and to make sure it's not the lawyer taking their own sweet time in advising the balance sum.

yeah2
so meaning no.3 is normal la for purchaser?

yes it's normal.