Making my own PCB connectors, but having trouble with high frequency signals
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!)