Here's a comparison and some numbers:
I run single crimped wires of length between 3" and 24", sometimes with multiple hops, and at 2 megabit data rate. The signal is 5V UART TTL, and the driver is an Atmega microcontroller (25 mA nominal pin current) with a 70 ohm current limiting resistor and a dozen or so 40 picoFarad MOSFET inputs on the other end.
This all works fine; oscilloscope shows the signal is decent, and communication works.
When I added a TVS diode with 2 nF capacitance across this bus, the signal degraded enough that I could not keep 2 Mbit data rate. The 3 dB filter frequency of a 70 ohm, 2 nF low-pass filter is about 600 kHz, IIRC, which would explain the signal degradation.
So, by comparison, you have one of three problems (or a combination):
1) The driver that is emitting the signal is not very strong. Some microcontrollers can only drive a few milliamps on their pins, for example, which translates into a high-impedance source.
2) The load you are driving is high capacitance somehow.
3) The wires you use add significant inductance.
The fix in 2) and 3) is to remove the cause. The fix in 1) is to re-drive the signal with a buffer or line driver (or perhaps a MOSFET gate driver, which can drive several amps!)
Note "bus extension" in this situation will only work if the device it's going to is purely passive (doesn't use the data lines) or you have a hub on the board.
0) Yes, it always takes absolutely ages to find the right parts and import them into your PCB tool. And there are always way too many of the wrong sort of parts.
1) "Shielded" connectors are the normal sort with a rectangular metal casing. Pick one of those. Not having a shield might give you interference problems (probably won't, but it's such a pain to diagnose). It also makes the mechanical connection more solid.
Gold plating is an affordable luxury that stops the contacts corroding. Less of an issue for contacts that are hidden away from fingers, but still nice to have if you care about longevity.
2) Through-hole is generally stronger than surface-mount. Some have extra plastic or metal mounting lugs to really lock it to the PCB.
3) The PCB-only connector requires that you get a PCB made of the required thickness. It sits a little more loosely in sockets, but it works and is very cheap. It also looks cheap. Connectorless USB on a PCB
Best Answer
One option is an Anderson Power Pole single-pin connector. These are a hermaphrodite 30 Amp single-pin connector that can be ganged together to make a connector that has as many pins as you want.
They are commonly seen in stage-lighting dimmer racks to patch the pins on the multi-pin Socapex connectors to the specific dimmer channel desired.
They are massively reliable and relatively inexpensive.
You can see them here. According to that page, the small ones are good for up to 45 Amps and all are rated for 600 Vac operation. They are UL listed and I know that I've seen them with CSA markings as well.
I really like these connectors - I've been using Power Pole connectors for decades and have always had great results with them. You will often see the larger double-pin connectors used in electric fork lifts for the battery connections.
[EDIT]
I just realized that you didn't specify what voltage and current you wanted.
There are large single-pin connectors available that are used for temporary AC power distribution panels. These go by the names "Camlock" & "Tweeco" and are available in current rating up to 200 Amps or more.
Have a chat with your local electricians about those if you need something that large.