Never have noted anything this dramatic when using quality coax cable and standard high gain output LNBFs. While I will agree that coax cables are best kept as short as possible, your reported losses seem extremely excessive.
Curious if the readings are being displayed with an uncalibrated STB or if it was measured with a calibrated meter. STBs have no calibrated reference and the actual loss may be significantly different than the 25% reduction in Signal Quality.
Typically, the Signal Quality is only a generalized expression of the Bit Error Rate after the FEC is applied. The method of determining the percentage is different for each developer, model and adjustable in the firmware.
I am sure that you would note that the loss is quite acceptable if measured as a signal to noise ratio with a calibrated meter.