Buy a US stock from Malaysia and you're making two bets at once. One is on the company. The other is on the ringgit versus the dollar, and you didn't get asked whether you wanted it.
Say you buy shares worth USD 1,000 when the rate is 4.40. That's RM4,400 out of your pocket. The stock climbs 5%, nice, but by the time you sell, the ringgit has strengthened to 4.20. Your USD 1,050 comes back as RM4,410. A 5% gain on the chart turned into roughly nothing in your bank account. Flip the movement and the ringgit's weakness can hand you a bonus you did nothing to earn. Either way, the exchange rate is sitting in your result whether you look at it or not.
Where the conversion actually happens
This depends on how you access the market american stock trading landscape. Some Malaysian-friendly brokers hold your cash in USD, so you convert once when you fund the account and again when you withdraw. Others convert automatically on every trade, which is convenient and sometimes expensive. Check whether the broker applies a markup to the rate. A margin of 0.5% or more over the mid-market rate looks harmless per transaction, then a few dozen trades later you're wondering where your profit went.
Bank transfers add another layer. Telegraphic transfers often carry a flat fee plus a rate that isn't the one you see on Google. Ask before sending, not after.
Timing the conversion (or not bothering)
People love trying to convert dollars only when the ringgit looks weak, and I get the temptation. It rarely works out cleanly. Currencies move on interest rate decisions, oil prices, trade news and plenty of things nobody predicted in advance. If you're investing over years, a sensible approach is to convert in chunks, monthly or quarterly, and let the rate average itself out. Boring, mostly effective.
Someone with a short-term trading style has a different problem. If you're in and out within days, the exchange-rate drift matters less than the round-trip cost of converting. Keep a dollar balance in your account and avoid sending money back to ringgit after every winner.
Costs beyond the rate
US stocks carry a 30% dividend withholding tax for non-US investors unless a treaty lowers it, and the US–Malaysia arrangement doesn't help much for individuals. So a dividend stock isn't quite paying what the yield says. Some brokers also charge custody or market-data fees, which are irritating for small accounts.
Track your results in ringgit as well as dollars. A spreadsheet with your entry rate, exit rate and fees takes twenty minutes to set up and tells you what actually landed in your account, which is the only number your rent cares about.