6hr layover in Heathrow

If immigration takes forty minutes, you only have five and one half hours. You'll need to be at the airport at least two hours before the flight. so you are now down to three and one half hours. Perhaps one half hour in transit each way and you are left with two and one half hours on site. If all goes as planned. Others may diffier, and that's ok, but I wouldn't want to be that rushed.

Not really.

FYI Uber isn’t universal….in fact once banned in London but these days…

https://www.theverge.com/2023/11/28/23979409/uber-london-black-cab-app-referrals-taxi-ltda

Same with Starbucks…..took alot of time and effort but Italy finally allowed them to open.

https://www.cnbc.com/amp/2022/10/17/starbucks-expanding-in-italy-despite-the-nations-intense-coffee-culture.html

I had a 6 hour layover there once as well. No, it's really not enough time to try to get into London and back without major stress, much less Windsor Castle. But you might be more daring. I tried to get as much of an England experience as I could just walking around the airport, looking around, taking a few photos, and buying a few things--I know, not quite as exciting.

Where are coming from as you arrive at Heathrow? The USA? Schengen country? Will you have to go through immigration / passport control to leave the international arrivals area. Sounds like you would. That would take time. What would the traffic be at the time of the day? How would the crowds be at the Castle, a popular tourist attraction. And then the hassle of going through security and possible passport control as you would be exiting the UK (I presume). I would be extremely nervous. Or at least thinking, the little time I would have to tour would be so minimal, is it worth the hassle and the risks. I would advise against doing so.

And my experience with Heathrow is that it’s manageable but vast. It can be a bit confusing and challenging. Perhaps to one who doesn’t frequent it, Heathrow can, dare I say, be a tad overwhelming.

As an earlier writer noted, explore the airport and its many shops. Or splurge a bit and buy a day pass to the airlines lounge. Just relax, have a bite to eat and maybe an adult beverage. Relaxing and recalling the events of a wonderful trip just concluded, or thinking of the exciting days ahead if you are just beginning your trip.

I wouldn't try it. Immigration and security can take awhile, and you need to be back at the airport well before your flight, especially if it's to another country. Windsor deserves a more leisurely visit than what you'll be able to give it.

I’ll be flying from Malta to the US. Arriving at 10:30 AM and have a 4:30 Delta flight to US. Sounds like it’s probably too complicated even in the off season!

If your fare class doesn't afford lounge privileges, perhaps buy a day pass to one ( even better if it has a shower).

The cost probably about what you would have spent for taxi or other ground transit, and a snack/lunch somewhere.

As a US citizen, you can use the egates at Heathrow to get through passport control.

Returning you would only have to go through security. There is no exit immigration in the UK.

However, I agree with the others....stay at the airport. The lounges are all airside so you wouldn't have to go through passport control.

What airline are you flying from Malta to London?

The general opinion is that you should remain at Heathrow--I agree.
For something to keep you busy: Heathrow has a wide variety of shops and restaurants.
A stroll around could be a lot of fun. Several bookstores, a small Harrods, various stores selling items with the British flag on them to take home as a gift or keep as a souvenir, and much more.

https://www.heathrow.com/at-the-airport/restaurants-a-z?type=restaurant
https://www.heathrow.com/at-the-airport/shops-a-z?type=store

It's possible that some walking will feel good, as you will have just come off one flight and will be getting on another flight where you'll just be sitting for several hours.

Short and sweet, No.