It has always been around 1 hour from north to south even when the MRT in Singapore was new. Stations and stops will add up taking more time compared to driving. Same thing happens in Malaysia.

I think JB in the future may have a special status, something like co-owned by Malaysia and Singapore. Malaysians entering JB need to go through immigration process, while travel between JB and Singapore will be free flowing.

Similar to the "foreign concession zone" in Shanghai during the early 1900s.

If majority of the JB citizens already working in Singapore, and Sultan and Singapore and the JB residents are all agreeable to have special laws which Singapore enforcement can control the JB citizens as far as various local civil and criminal laws are concerned. Essentially JB laws will be more similar to Singapore than Malaysia due to this special "status". Just like HK and PRC mainland have different laws, while still belonging to PRC. JB citizens who commit crime in Singapore will still be easily arrested by Singapore in JB. To escape, they have to go through immigration at JB border to enter Malaysia.

Of course many parties will object. But certain situations may arise that they find it acceptable, if JB become way more richer than Malaysia and is no longer really "Malaysia" since most of them spend their time in Singapore anyway.