This gist shows the difference of using locator.fill in playwright when small strings are separated by a single space compared to being separated by a newline. Also, the difference between the three major browsers is staggering!

textarea_filler_weirdness.py

"""Test Playwright in entering text to textarea element.
"""
import time
from playwright.sync_api import sync_playwright, Page, expect


T_O = 300000  # Five minutes


def textarea_fill_separated_by(page: Page, sep: str):
    """Test entering data to textarea when separated by blanks and newlines.
    """
    for i in range(1000, 4001, 1000):
        joined_string = sep.join(['some text'] * i)
        start = time.time()
        page.get_by_label('Message:').fill(joined_string, timeout=T_O)
        end = time.time()
        combinator = 'blanks' if sep == ' ' else 'newlines'
        delta = round((end - start), 3)
        print(f'Entering {i} text separated by {combinator} took {delta}'
              ' seconds')
        page.locator('css=textarea#message-text').fill('')


with sync_playwright() as p:
    for browser in [p.firefox.launch(headless=False),
                    p.webkit.launch(headless=False),
                    p.chromium.launch(headless=False)]:
        print('\n')
        print(f'Browser is: {browser.browser_type.name}')
        page = browser.new_page()
        page.goto("https://demoblaze.com/index.html")
        # Expect a title
        expect(page).to_have_title('STORE')
        #
        page.get_by_role("link", name="Contact").click()
        #
        textarea_fill_separated_by(page, ' ')
        textarea_fill_separated_by(page, '\n')

        browser.close()

Firefox has no problem with multiline texts, four thousand lines are filled under a second,
webkit starts to choke a bit at 3000 lines but.
chromium takes well over a minute to enter 4000 lines to a textarea element!