When visiting new countries in most of the world the standard advice is to move slowly and concentrate as much time as possible in each of the biggest and most famous cities. For example, on a first trip to Europe people will tell you to spend a full week in London, a full week in Paris, and a full week in Rome if you’ve got that long. But in the southern countries of Asia (India, Thailand, Vietnam, Indonesia, Philippines etc.), the best strategy is nearly the opposite.
The huge and famous cities in this region, including Mumbai, Bangkok, Saigon, Jakarta, Colombo, Manila and many others, are generally overcrowded, expensive, frustrating, and short on charm. In their own way each is worth seeing, but unlike most other regions it’s best to plan to move through in two days or less, and then move on to beaches or ruins or temples or villages.
The short version
South Asia has a great number of huge cities that are practically bursting at the seams. Each has at least a few interesting museums or temples, plus that country’s best nightlife and shopping, but most people will enjoy their stay more if they keep the city visits short.
By all means, if a huge city has a long list of things that sound very worthwhile to you, then plan to stay as long as you need to in order to see them. But otherwise don’t hang around just because it’s a city you’ve heard of.
6 Reasons not to linger in south Asia's largest cities
1 – Big cities are expensive, and poor value compared to everywhere else

Big cities are expensive all over the world and south Asia is no exception. The difference is that Paris, Rome, Amsterdam, and New York City are undisputed highlights of their countries, while Delhi, Saigon, and Dhaka are little more than overly crowded landing zones that become frustrating after only a couple days. Visit the big south Asian cities for sure, but make a list of the things you really care about, and knock those out in as few days as you can.
2 – The main checklist attractions are generally weak

The point of this one is that you shouldn’t just schedule a long stay in a huge city and assume that you’ll find an endless list of wonderful things once you are there, like you might when visiting London. Buy a guidebook or do online research to determine which are the things that sound most worthwhile, and then start looking for smaller cities and villages to visit next.
3 – Big cities tend to be less exotic and very familiar

I still consider myself a fan of urban living and the diversity it usually brings, but in south Asia the charm is generally missing. The large cities seem to get more generic as they grow, and they get more expensive at the same time.
4 – Getting around in the big cities can crush your travel spirit

So the alternatives are a mix of taxis (with drivers who rarely speak English), tuk-tuk drivers with their own agenda, buses, walking in the scorching heat, and a few subway or monorail systems that have spotty coverage. Combine the heat with the often-correct feeling that many people are trying to rip you off with each ride, and getting around these giant cities gets very frustrating.
5 – The heat and/or rain can be punishing in cities

The symphony of air conditioners dripping condensation on your head as you dodge motorbikes on the sidewalk can make things very uncomfortable, with no breeze to help out. Once you are in the open air or on a beach the heat can feel pleasant or at least bearable. Cities also have a tougher time dealing with the huge thunder storms in season, and even major streets can flood for days at a time.
6 – People are friendlier and more accessible in smaller towns

When people reflect on a trip the one thing that usually seems to stand out to them are the people they’ve met rather than the buildings they’ve seen. If you travel to places even a little off the common trail, you’ll usually find locals who are fascinated with you, very welcoming, and happy to make your visit the best it can be. Even with a limited common language, you’ll feel a greeting and interest that you just can’t get in large cities.
Yes to all of the above, but the cities have the best variety and availability of food.
Fakename, you have a point about the food, but I think it’s still possible to get all the main famous dishes from each country in the smaller cities, and usually cheaper as well. The cities are home to more international restaurants and high-end restaurants for sure. -Roger