Send serial commands through IDE while RX pin is in use
Regarding “I have connected my LED strip (WS2812B LEDs) to the RX port”:  If you connect an LED strip to the RX pin, there's a good chance you won't be able to read serial data reliably on that pin, and may have trouble loading new sketches.  It makes sense to connect the LEDs to some pin other than RX.

Regarding “If I open up the serial monitor, how do I send something to it”:  Ordinarily, one uses the serial monitor to receive data from an attached MCU or to send data to the MCU. (An MCU is a microcontroller – eg an Arduino or ESP8266.)  In the sketch on the MCU, use Serial.write() to send binary data to the monitor, and Serial.print() to send text and formatted numbers to the monitor.

If you want to send serial data rapidly from your host computer to the MCU, write a C or Python (etc) program that opens a serial port, marshals the data, and sends it.  In the  sketch on the MCU, use Serial.available to test if characters are available to read, and Serial.read to read a character.  For faster transmission, you can send binary data instead of ASCII, but would need to read up on that before attempting it. 

For fast data transmission, aim for either high data rates or strong data compression.  For example, you might make up a protocol that only sends colors for pixels that are going to change; or might have high-level commands that tell the MCU how to generate a set of data, via color end points, step rates, and numbers of steps.

In general, wifi data rates are somewhat higher than serial data rates.  If that is not what you are experiencing, perhaps review the article Found a way to upload data VERY fast by wifi from github.com/esp8266.



Edit 1:
Regarding “Nothing is displayed in the serial monitor” [from code like if (Serial.available()) { Serial.write(Serial.read()); }]:  I'm assuming that the sketch in use has a setup() routine containing a Serial.begin(somedatarate) statement, with the same data rate selected in the serial monitor as in the Serial.begin() statement.   I'm also assuming the sketch has a loop() routine containing the if (Serial.available()) { Serial.write(Serial.read()); } code.

I'm also assuming the sketch was properly downloaded, and then the serial monitor was opened, using the USB-serial port the ESP8266 is attached to, and then you typed some text into the text entry box of the serial monitor and pressed or clicked Enter to send the text to the ESP8266, with a proper setting for the end-of-line character.  

If all those assumptions are correct, but nothing shows up in reply to the text you send, then it seems likely some hardware or software issue is occurring.  For example, when the Serial data structure is set up (when the sketch begins running) it might be selecting an incorrect Rx pin.  Or something may be loading down the Rx pin.  To find out if either of those is the case, use an oscilloscope (if you have one) to look at what's arriving at different ESP8266 pins when you send text from the  serial monitor.  If no scope, connect a 5K-10KΩ resistor to an LED and use that as a logic probe.  One end of the resistor connects to a probe; the other end goes to the LED anode; and the LED cathode is grounded.  Test if that's working by touching the probe to +3.3V.  When the probe appears to be working ok, use it to check for brief flashes of data at different Rx pins on the ESP8266 when you send data from the serial monitor.  If no data gets to the ESP8266, it can't echo it.  (Note, I'm not sure how your NodeMCU ESP8266 handles serial-via-USB data; my notion that serial data should be visible on some Rx pin might be wrong.)  

If it appears data is getting to some pin but isn't showing up as available, you might try opening software serial on that pin.  Some examples appear in code in a serial monitor note at github.com/esp8266.  I don't know enough about NodeMCU / ESP8266 to say what to do if probing and software serial don't help.