Looping in Python Slowing Down with each Iteration - python-2.7

Question

The following code produces the desired result, but the root of my issue stems from the process slowing down with each append. In the first 100 pages or so, it runs at less than one second per page, but by the 500s it gets up to 3 seconds and into the 1000s it runs at about 5 seconds per append. Are there suggestions for how to make this more efficient or explanations for why this is just the way things are?
import lxml
from lxml import html
import itertools
import datetime
l=[]
for pageno in itertools.count(start=1):
time = datetime.datetime.now()
url = 'http://example.com/'
parse = lxml.html.parse(url)
try:
for x in parse.xpath('//center'):
x.getparent().remove(x)
x.clear()
while x.getprevious() is not None:
del x.getparent()[0]
for n in parse.xpath('//tr[#class="rt"]'):
l.append([n.find('td/a').text.encode('utf8').decode('utf8').strip()\
,n.find('td/form/p/a').text.encode('utf8').decode('utf8').strip()\
,n.find('td/form/p/a').attrib['title'].encode('utf8').decode('utf8').strip()]\
+[c.text.encode('utf8').decode('utf8').strip() for c in n if c.text.strip() is not ''])
n.clear()
while n.getprevious() is not None:
del n.getparent()[0]
except:
print 'Page ' + str(pageno) + 'Does Not Exist'
print '{0} Pages Complete: {1}'.format(pageno,datetime.datetime.now()-time)
I have tried a number of solutions, such as disabling the garbage collector, writing one list as a row to file instead of appending to a large list, etc. I look forward to learning more from potential suggestions/answers.