just to let others know.
today i did some testing with about 6 dongles. while the differences between the dongles (3 nooelec and 3 rtl-sdr.com) are small i stumbled over something i couldn’t believe. i have lots of pigtail-cables sma/sma and sma/mcx - all nearly identical looking and about 10-15cm long. but there are reasonable differences in message-rates (about 200/sec). with the same setup and dongle i can get 1.300 or 1.500 msg/sec depending on pigtail. and no - not a question of gain and attenuation. this was my first thought - so i tested this too - but no - some pigtails are pigs …
maybe you should check your pigtails too