Of course you can configure LimeSDR via LimeSuiteGUI. Rx sample rate is set by configuring CLKGEN (to set ADC clock) and decimation (RxTSP). I don’t know if you have some chip configuration, but just to see how the sample rate is set, you can load initial configuration by clicking ‘Default’ button. After that go to CLKGEN tab and set CLK_H to 16 MHz and click ‘Calculate’ (ADC clock will be 4 times lower so 4MHz). Default decimation is 2^1, so final sample rate will be 2Msample/s. You can open FFTviewer and press ‘Start’. Data rate should be ~6MB/s as FFTviewer uses 12bit samples.
You can increase decimation in RxTSP tab by changing ‘HBD ratio’ (you need to click ‘Calculate’ in CLKGEN tab after changing decimation).