pySerial reading data is delayed

I am reading serial data from an esp32 with pySerial. The esp32 is sending the data quite fast (~every 10ms). If I try to read out the data with pyserial, i takes really long.

Without a specified timeout for pySerial class, it takes around 3s to read out my command string and I dont know what it is waiting for. Furthermore, because of the 3s, the buffer is getting bigger and bigger.

With a timeout and with reducing the sending frequency of the esp32 to 200ms, it works. But if i use a too small timeout it won't read out my whole command string I send with the esp32. And i have to adjust the timeout to the sending frequency of the esp.

What irritates me, is that the 'read_until()' method is taking 3s to read until my seperator char. But if the method stops earlier because of the timeout, it already fetched the whole serial data string.

class USBReader:
    def __init__(self) -> None:
        self.readerThread = Thread(target=self.read_usb)
        self.ser = serial.Serial(port='COM3', baudrate=500000, timeout=0.3)

    def read_usb(self):
        while True:
            start = datetime.datetime.now()
            self.ser.flushOutput()
            try:
                start = datetime.datetime.now()
                command_ = self.ser.read_until(b';').decode("UTF-8")
                end = datetime.datetime.now()
                command = command_.replace("\n", "").replace("\r", "")
                print(command)
                print(end - start)
            except UnicodeDecodeError:
                print("error")

EDIT:

I found out the problem was with my two Threads i used. One Thread is for reading the data and the second one is for processing it. The processing was done inside a while loop, which was using all the RAM?. I inserted a sleep of 0.01s and everythings works fine