The following 35 files account for 2/3rds all of HTML parse-time when running ./wpt manifest -r under Python 3.8, and HTML parsing is known to be a bottleneck there; the first column is time to parse in nanoseconds, the second column is the path:
135653604 css/compositing/test-plan/test-plan.html
176767866 css/CSS2/floats-clear/clear-on-parent-with-margins.html
193460915 css/css-flexbox/interactive/flexbox_interactive_order-transitions.html
197030005 encoding/legacy-mb-tchinese/big5/big5_chars_extra.html
221161410 html/browsers/offline/appcache/appcache-iframe.https.html
224716724 mathml/relations/css-styling/mathvariant-auto-ref.html
239263995 css/vendor-imports/mozilla/mozilla-central-reftests/text3/segment-break-transformation-rules-004.html
579512274 encoding/legacy-mb-japanese/shift_jis/sjis_chars-csshiftjis.html
580270155 encoding/legacy-mb-japanese/shift_jis/sjis_chars-windows-31j.html
581770313 encoding/legacy-mb-japanese/shift_jis/sjis_chars.html
582403778 encoding/legacy-mb-japanese/shift_jis/sjis_chars-sjis.html
582967171 encoding/legacy-mb-japanese/iso-2022-jp/iso2022jp_chars-csiso2022jp.html
587603832 encoding/legacy-mb-japanese/shift_jis/sjis_chars-ms932.html
746397201 encoding/legacy-mb-japanese/shift_jis/sjis_chars-ms_kanji.html
746419899 encoding/legacy-mb-japanese/shift_jis/sjis_chars-shift-jis.html
752400005 encoding/legacy-mb-japanese/iso-2022-jp/iso2022jp_chars.html
776815743 encoding/legacy-mb-japanese/shift_jis/sjis_chars-x-sjis.html
1061015751 encoding/legacy-mb-japanese/euc-jp/eucjp_chars.html
1150058943 encoding/legacy-mb-tchinese/big5/big5_chars-cn-big5.html
1221893189 encoding/legacy-mb-japanese/euc-jp/eucjp_chars-cseucpkdfmtjapanese.html
1242618515 encoding/legacy-mb-japanese/euc-jp/eucjp_chars-x-euc-jp.html
1283325328 encoding/legacy-mb-tchinese/big5/big5_chars.html
1303526005 encoding/legacy-mb-tchinese/big5/big5_chars-big5-hkscs.html
1304694399 encoding/legacy-mb-tchinese/big5/big5_chars-x-x-big5.html
1326822724 encoding/legacy-mb-tchinese/big5/big5_chars-csbig5.html
1339532636 encoding/legacy-mb-korean/euc-kr/euckr_chars-ksc5601.html
1350751318 encoding/legacy-mb-korean/euc-kr/euckr_chars-windows-949.html
1358561832 encoding/legacy-mb-korean/euc-kr/euckr_chars-iso-ir-149.html
1495772737 encoding/legacy-mb-korean/euc-kr/euckr_chars-ks_c_5601-1989.html
1498091000 encoding/legacy-mb-korean/euc-kr/euckr_chars-korean.html
1501368952 encoding/legacy-mb-korean/euc-kr/euckr_chars-cseuckr.html
1512773076 encoding/legacy-mb-korean/euc-kr/euckr_chars-ksc_5601.html
1523776956 encoding/legacy-mb-korean/euc-kr/euckr_chars-csksc56011987.html
1532068847 encoding/legacy-mb-korean/euc-kr/euckr_chars-ks_c_5601-1987.html
1542689788 encoding/legacy-mb-korean/euc-kr/euckr_chars.html