Mobjack

I don't think any computer can handle 20,000 parts based off math. I believe that the way the physics scales in KSP, every time you double the part count, you need 4x the computing power. Multicore computers and GPU calculations will help, but it wouldn't be an order of magnitude change. You are going to hit a new wall and it will be likely sooner than you think.

The reasons your designs are inefficient is because they don't suck. The secret to making an efficient SSTO is intakes, intakes and more intakes. Air intakes are extremely overpowered for SSTO's. With enough intake, you can run your jet engines at 35,000 meters going 2300 m/s. All it takes is a little more dV to actually get into orbit. If you want to make something efficient, don't build something that looks good and could work in real life. You just need to find a way to attach as many intakes as possible.

You need to upgrade your patience. Either wait through the lag or wait for Squad to implement multithreading for physics calculations. The way physics scale, doubling the number of parts requires 4x the processing power. Even if you are able to find a processor that is 50% faster, you will only be able to increase the part count by 22%.

I don't really find making a few orbits to graze the atmosphere to be fun. Sometimes I just want to land and not waste the extra ten minutes doing multiple passes. It also isn't that easy to land without heat shielding with deadly reentry especially without using multiple orbits. It can be fun for experienced players and I use it, but it will be frustrating for new players or those who just like building crazy ships. Even simple SSTO designs are hard to land with deadly reentry since the wings aren't protected by shielding. If they implement the feature in KSP, it does have to be carefully balanced to take these factors into consideration.

The re-entry heat could be implemented so it isn't a major factor for ships in LKO, but becomes an issue for ships coming in faster.

The MK3 cockpit looks good if you use the MK2 adaptor. Here is my 3 person SSTO workhorse: I agree that the MK3 fuselage is ugly and I never use that. Maybe if that is a cargo bay or a crew storage container, it would make sense. Another problem is that the MK2 fuselage doesn't match the MK2 cockpit. The MK2 fuselage looks good, but it needs a cockpit that can seamlessly match.

Just do it. Try to hit every moon, but if you can't, hit as many as you can before landing on Laythe. You are sending a probe to explore the unknown and part of the fun is figuring the stuff out on your own.

I think part of the reason Squad doesn't have an dV indicator is that it is very hard to implement. Once you have complex staging or docking involved, the calculations aren't going to be accurate.

I would say that even if my design wasn't exploitative according to the rules, I did exploit every advantage I can get. When going up at a high attitude, I start shutting down engines as they run out of air, which sends more air to other engines in an unrealistic manner. You can still do some air hogging, but it is not quite as exploitative. A neat trick with the RAPIER engines is that when you run out of air, you only switch two of them to a closed cycle. This sends more air to your air breathing RAPIER engines so you don't have to run a 100% closed cycle. Squad really needs to balance the air intakes better in KSP to make the challenges more fair. I like making efficient vehicles, but with SSTO challenges, it just becomes an exercise of adding as much intakes as possible which takes the fun out of it.

Why was the Angry Beetle in the exploitation category? My design had two air intakes in front attached to each engine with a bi-coupler. The intake to engine ratio is less than 2. Here is a better look of the setup:

I had a complicated craft sitting on my launchpad and it slowed down all my planes on the runway. Once I recovered that rocket to clear the launchpad, the game sped up. Make sure you don't have any debris or extra ships lying around because they can lag you.

I only had real issues about clipping through the surface landing on Eve, but that planet has the highest gravity. I landed on every other planet with no problem. Sometimes my Kerbal will sink down to his waist on Kerbin's grasslands, but he still survives. If you are concerned, quicksave before landing. The one graphic setting you might want to set higher is the render quality to good. Lower rendering settings don't display your shadow at a distance which makes it hard to gauge your distance landing. Most laptops are crap for running games. They are designed more for portability than gaming, but you usually can make the games playable if you set the setting down. I play KSP on a Mac Mini with integrated graphics and still have fun. I don't know how bad your CPU is but it shouldn't be an issue with small ships. The way physics scales, every time you double your ship parts, you need 4x the processing power which creates a big bottleneck for the huge complex ships, but it also means that any old thing can render something small.

If it is always in the yellow then the problem is with your GPU. The CPU bottleneck occurs when building big ships, but if even a a 20 part ship lags, then it is your graphics. Literally set the settings to the lowest detail possible. I play with the terrain settings set to low, and while there is some clipping, it doesn't affect my gameplay. It is worth it to turn it down because it is the biggest killer of frame rates for me.

Here is my latest attempt with major improvements in both efficiency and style. I call it the Angry Beetle. 16 Intakes, 5 Turbojets and 4 RAPIERs. ((1659+2695) / (3240+3960)) * 100 = 60.5

The top stages should be small and the bottom ones should be big. As a general rule, each stage should be at least twice as big as the previous.